feat: wire ACME EAB into account registration + ZeroSSL auto-fetch

EAB credentials (KID + HMAC) were defined in the ACME connector config
but never wired into the acme.Account registration call. This fixes the
dead code and adds automatic EAB credential fetching for ZeroSSL — when
the directory URL is detected as ZeroSSL and no EAB credentials are
provided, certctl calls ZeroSSL's public API to get them automatically.

Changes:
- Wire EABKid/EABHmac into acme.Account.ExternalAccountBinding
- Add isZeroSSL() detection and fetchZeroSSLEAB() auto-fetch
- Add CERTCTL_ACME_EAB_KID/CERTCTL_ACME_EAB_HMAC env vars to main.go
- Add 13 ACME connector tests (config validation, EAB decode, ZeroSSL
  auto-EAB with mock servers, URL detection)
- Update docs: README, architecture, connectors, demo-advanced,
  testing-guide with EAB/auto-EAB documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture.md

@@ -77,7 +77,7 @@ flowchart TB
 
     subgraph "Issuer Backends"
         CA1["Local CA\n(crypto/x509, sub-CA)"]
-        CA2["ACME\n(HTTP-01 + DNS-01 + DNS-PERSIST-01)"]
+        CA2["ACME\n(HTTP-01 + DNS-01 + DNS-PERSIST-01)\n(EAB, ZeroSSL auto-EAB)"]
         CA3["step-ca\n(/sign API)"]
         CA4["OpenSSL / Custom CA\n(script-based)"]
         CA6["Vault PKI\n(planned)"]
@@ -563,7 +563,7 @@ type Connector interface {
 }
 ```
 
-Built-in issuers: **Local CA** (self-signed or sub-CA mode using `crypto/x509`), **ACME v2** (HTTP-01, DNS-01, and DNS-PERSIST-01 challenges, compatible with Let's Encrypt, Sectigo, and any ACME-compliant CA), **step-ca** (Smallstep private CA via native /sign API with JWK provisioner auth), and **OpenSSL/Custom CA** (script-based signing delegating to user-provided shell scripts). The ACME connector uses `golang.org/x/crypto/acme`, generates an ECDSA P-256 account key, handles account registration with ToS acceptance, order creation, challenge solving (HTTP-01 via built-in server, DNS-01 via script-based hooks, DNS-PERSIST-01 via standing TXT records with auto-fallback to DNS-01), order finalization, and DER-to-PEM chain conversion. The interface also includes `GetCACertPEM(ctx)` for CA chain distribution (used by the EST server's `/cacerts` endpoint).
+Built-in issuers: **Local CA** (self-signed or sub-CA mode using `crypto/x509`), **ACME v2** (HTTP-01, DNS-01, and DNS-PERSIST-01 challenges, compatible with Let's Encrypt, ZeroSSL, Sectigo, Google Trust Services, and any ACME-compliant CA), **step-ca** (Smallstep private CA via native /sign API with JWK provisioner auth), and **OpenSSL/Custom CA** (script-based signing delegating to user-provided shell scripts). The ACME connector uses `golang.org/x/crypto/acme`, generates an ECDSA P-256 account key, handles account registration with ToS acceptance and optional External Account Binding (EAB) for CAs that require it (ZeroSSL, Google Trust Services, SSL.com), order creation, challenge solving (HTTP-01 via built-in server, DNS-01 via script-based hooks, DNS-PERSIST-01 via standing TXT records with auto-fallback to DNS-01), order finalization, and DER-to-PEM chain conversion. For ZeroSSL, EAB credentials are auto-fetched from ZeroSSL's public API when the directory URL is detected as ZeroSSL and no EAB credentials are provided — zero-friction onboarding with no dashboard visit required. The interface also includes `GetCACertPEM(ctx)` for CA chain distribution (used by the EST server's `/cacerts` endpoint).
 
 ### Target Connector
 

docs/connectors.md

@@ -202,11 +202,35 @@ DNS-PERSIST-01 configuration:
 
 The present script creates a TXT record at `_validation-persist.<domain>` with the value `letsencrypt.org; accounturi=https://acme-v02.api.letsencrypt.org/acme/acct/<your-id>`. This record is permanent — no cleanup script is needed.
 
+ZeroSSL configuration (requires External Account Binding):
+```json
+{
+  "directory_url": "https://acme.zerossl.com/v2/DV90",
+  "email": "admin@example.com",
+  "eab_kid": "your-zerossl-eab-kid",
+  "eab_hmac": "your-zerossl-eab-hmac-base64url"
+}
+```
+
+ZeroSSL, Google Trust Services, and SSL.com require External Account Binding (EAB) for ACME account registration. For most CAs, get your EAB credentials from the CA's dashboard and provide them via `eab_kid` and `eab_hmac`. The HMAC key must be base64url-encoded (no padding). CAs that don't require EAB (Let's Encrypt, Buypass) ignore these fields.
+
+**ZeroSSL auto-EAB:** When the directory URL points to ZeroSSL and no EAB credentials are provided, certctl automatically fetches them from ZeroSSL's public API (`api.zerossl.com/acme/eab-credentials-email`) using your configured email address. No dashboard visit required — just set the directory URL and email, and it works. This is the same approach used by Caddy and acme.sh.
+
+Minimal ZeroSSL configuration (auto-EAB):
+```json
+{
+  "directory_url": "https://acme.zerossl.com/v2/DV90",
+  "email": "admin@example.com"
+}
+```
+
 DNS hook scripts receive these environment variables: `CERTCTL_DNS_DOMAIN` (domain being validated), `CERTCTL_DNS_FQDN` (full record name — `_acme-challenge.<domain>` for dns-01, `_validation-persist.<domain>` for dns-persist-01), `CERTCTL_DNS_VALUE` (TXT record value), `CERTCTL_DNS_TOKEN` (ACME challenge token). The present script must create the TXT record and exit 0; the cleanup script removes it (dns-01 only).
 
 Environment variables for the default ACME connector:
 - `CERTCTL_ACME_DIRECTORY_URL` — ACME directory URL
 - `CERTCTL_ACME_EMAIL` — Contact email for account registration
+- `CERTCTL_ACME_EAB_KID` — External Account Binding Key ID (required by ZeroSSL, Google Trust Services, SSL.com)
+- `CERTCTL_ACME_EAB_HMAC` — External Account Binding HMAC key (base64url-encoded)
 - `CERTCTL_ACME_CHALLENGE_TYPE` — `http-01` (default), `dns-01`, or `dns-persist-01`
 - `CERTCTL_ACME_DNS_PRESENT_SCRIPT` — Path to DNS record creation script (dns-01 and dns-persist-01)
 - `CERTCTL_ACME_DNS_CLEANUP_SCRIPT` — Path to DNS record cleanup script (dns-01 only, not used by dns-persist-01)

docs/demo-advanced.md

@@ -11,6 +11,7 @@ This demo goes beyond browsing pre-loaded data. You'll create a team, register a
 2. [How the pieces fit together](#how-the-pieces-fit-together)
 3. [Alternative Issuers Reference](#alternative-issuers-reference)
    - [Sub-CA Mode](#sub-ca-mode-local-ca-chained-to-enterprise-root)
+   - [ACME with ZeroSSL](#acme-with-zerossl-auto-eab)
    - [ACME with DNS-01 Challenges](#acme-with-dns-01-challenges-wildcard-certificates)
    - [ACME with DNS-PERSIST-01](#acme-with-dns-persist-01-zero-touch-renewals)
    - [step-ca (Smallstep Private CA)](#step-ca-smallstep-private-ca)
@@ -96,6 +97,27 @@ docker compose -f deploy/docker-compose.yml restart server
 
 The CA key can be RSA, ECDSA, or PKCS#8 format. The connector validates that the certificate has `IsCA=true` and `KeyUsageCertSign`.
 
+### ACME with ZeroSSL (Auto-EAB)
+
+ZeroSSL is a free ACME CA that requires External Account Binding (EAB) for account registration. certctl auto-fetches EAB credentials from ZeroSSL's public API when the directory URL is detected as ZeroSSL and no EAB credentials are provided — you just need an email address:
+
+```bash
+# Minimal config — certctl auto-fetches EAB credentials from ZeroSSL
+export CERTCTL_ACME_DIRECTORY_URL="https://acme.zerossl.com/v2/DV90"
+export CERTCTL_ACME_EMAIL="ops@example.com"
+```
+
+No dashboard visit, no manual EAB credential copy-paste. certctl calls `api.zerossl.com/acme/eab-credentials-email` with your email, gets back a KID + HMAC key, and uses them for ACME account registration automatically.
+
+If you already have EAB credentials (e.g., from the ZeroSSL dashboard or for other CAs like Google Trust Services or SSL.com), you can provide them explicitly:
+
+```bash
+export CERTCTL_ACME_DIRECTORY_URL="https://acme.zerossl.com/v2/DV90"
+export CERTCTL_ACME_EMAIL="ops@example.com"
+export CERTCTL_ACME_EAB_KID="your-key-id"
+export CERTCTL_ACME_EAB_HMAC="your-base64url-hmac-key"
+```
+
 ### ACME with DNS-01 Challenges (Wildcard Certificates)
 
 For Let's Encrypt or other ACME providers with wildcard support:

docs/testing-guide.md

@@ -1490,6 +1490,26 @@ curl -s -H "$AUTH" "$SERVER/api/v1/issuers/iss-acme-le" | jq '{id, type}'
 
 ---
 
+**Test 6.2.3 — Configure ACME with External Account Binding (ZeroSSL)**
+
+Edit `deploy/docker-compose.yml` to set EAB environment variables:
+- `CERTCTL_ACME_DIRECTORY_URL: https://acme.zerossl.com/v2/DV90`
+- `CERTCTL_ACME_EAB_KID: your-zerossl-kid`
+- `CERTCTL_ACME_EAB_HMAC: your-base64url-hmac-key`
+
+Restart and verify the issuer accepts the config:
+
+```bash
+curl -s -H "$AUTH" "$SERVER/api/v1/issuers/iss-acme-prod" | jq '{id, type}'
+```
+
+**What:** Verifies that ACME issuers read External Account Binding credentials from environment variables.
+**Why:** ZeroSSL, Google Trust Services, and SSL.com require EAB for ACME account registration. Without EAB, account creation fails and no certificates can be issued from these CAs.
+**Expected:** HTTP 200. ACME issuer functional with EAB credentials loaded.
+**PASS if** HTTP 200 and issuer responds. **FAIL** if 500 or startup errors related to EAB.
+
+---
+
 ## Part 7: Target Connectors & Deployment
 
 **What this validates:** CRUD for deployment targets, including type-specific configuration for all 5 target types.