feat(security): bodyLimit on noAuth + security headers + encryption-key validation (H-1 master)

Closes three 2026-04-24 audit findings (all P2):
  - cat-s5-4936a1cf0118: noAuthHandler chain accepted arbitrary-size
    bodies (EST simpleenroll, SCEP, PKI CRL/OCSP, /health, /ready).
    Memory exhaustion vector without HTTP-layer auth gatekeeping.
  - cat-s11-missing_security_headers: zero security headers on any
    response. Clickjacking, MIME-sniffing, untrusted-origin resource
    loads against the dashboard and API.
  - cat-r-encryption_key_no_length_validation: CERTCTL_CONFIG_ENCRYPTION_KEY
    accepted with any non-empty value including a single character.
    PBKDF2-SHA256 (100k rounds) does not compensate for low-entropy
    passphrases at scale (CWE-916, CWE-329).

Changes:
- cmd/server/main.go::noAuthHandler chain — added bodyLimitMiddleware
  + securityHeadersMiddleware. Same default cap as authed surface
  (1MB via CERTCTL_MAX_BODY_SIZE), same 413 on overflow.
- cmd/server/main.go::middlewareStack (authed) — added
  securityHeadersMiddleware before corsMiddleware.
- internal/api/middleware/securityheaders.go (new) — SecurityHeaders
  middleware + SecurityHeadersDefaults() with conservative defaults:
  HSTS 1y+includeSubDomains, X-Frame-Options DENY, X-Content-Type-
  Options nosniff, Referrer-Policy no-referrer-when-downgrade, CSP
  default-src 'self' + img/data + style 'unsafe-inline' (Tailwind/Vite
  needs it; scripts still 'self' only) + connect 'self' + frame-
  ancestors 'none'. Operators behind a customising reverse proxy can
  disable any header by setting its config field to empty.
- internal/config/config.go::Validate() — enforce minEncryptionKeyLength
  = 32 bytes when CERTCTL_CONFIG_ENCRYPTION_KEY is set. Empty stays
  accepted (downstream fail-closed sentinel handles it). Structured
  error names the env var, the actual length, the required minimum,
  and the canonical generation command (`openssl rand -base64 32`).

Tests:
- internal/api/middleware/securityheaders_test.go (new) — 4 cases
  (defaults present, empty value disables single header, override
  applied, headers on 4xx/5xx).
- internal/config/config_test.go — 5 new cases for the encryption-key
  length check (empty accepted, 1-byte rejected, 31-byte rejected at
  boundary, 32-byte accepted, 44-byte realistic operator key accepted).

Documentation:
- CHANGELOG.md — H-1 section above D-2 under [unreleased] with
  Breaking-change callout (operators with low-entropy keys must rotate
  before upgrade).
- coverage-gap-audit-2026-04-24-v5/unified-audit.md — Live Tracker
  25/47 → 33/47, P1 14/14 (zero remaining), P2 11/27 → 16/27. Three
  H-1 findings flipped + closed-bundle row added.

Verification:
- go build ./... — clean
- go vet ./... — clean
- golangci-lint v2.11.4 run ./... — 0 issues
- go test ./internal/api/middleware/... — pass (incl. 4 new
  SecurityHeaders cases)
- go test ./internal/config/... — pass (incl. 5 new EncryptionKey
  cases)
- tsc --noEmit (frontend) — clean
- All sibling guardrails (S-1 / G-3 / D-1 / D-2 / B-1 / L-1) still pass

Audit findings closed:
- cat-s5-4936a1cf0118 (P2)
- cat-s11-missing_security_headers (P2)
- cat-r-encryption_key_no_length_validation (P2)

Breaking change:
- Operators with CERTCTL_CONFIG_ENCRYPTION_KEY shorter than 32 bytes
  must rotate before upgrade. Generate via `openssl rand -base64 32`.

Deferred follow-ups:
- Weak-key dictionary check (reject password123, common ASCII patterns)
  — adds operational friction with low marginal entropy gain at the
  32-byte minimum.
- CSP 'unsafe-inline' for styles — required for Tailwind/Vite
  per-component <style> blocks; removing requires HTML report or
  component refactor outside H-1 scope.
- Permissions-Policy header — dashboard uses no advanced browser APIs
  (camera, mic, geolocation); deferred until a real consumer needs it.

internal/config/config.go

@@ -1176,6 +1176,26 @@ func (c *Config) Validate() error {
 		return fmt.Errorf("server TLS cert/key pair invalid (cert=%q key=%q): %w — refuse to start (HTTPS-only; see docs/tls.md)", c.Server.TLS.CertPath, c.Server.TLS.KeyPath, err)
 	}
 
+	// H-1 closure (cat-r-encryption_key_no_length_validation): if
+	// CERTCTL_CONFIG_ENCRYPTION_KEY is set, enforce a minimum length of
+	// 32 bytes. Pre-H-1 the field was accepted with any non-empty value
+	// — including a single character — and PBKDF2-SHA256 (100k rounds)
+	// alone does not compensate for low-entropy passphrases at scale
+	// (CWE-916 Use of Password Hash With Insufficient Computational
+	// Effort + CWE-329 Generation of Predictable IV with CBC Mode).
+	// 32 bytes ≈ 256 bits when generated via `openssl rand -base64 32`,
+	// matching the AES-256-GCM key size the passphrase derives. An
+	// empty key remains accepted — the fail-closed sentinel
+	// crypto.ErrEncryptionKeyRequired triggers downstream when an
+	// empty key is asked to encrypt or decrypt sensitive config.
+	const minEncryptionKeyLength = 32
+	if c.Encryption.ConfigEncryptionKey != "" && len(c.Encryption.ConfigEncryptionKey) < minEncryptionKeyLength {
+		return fmt.Errorf(
+			"CERTCTL_CONFIG_ENCRYPTION_KEY too short (%d bytes; minimum %d). Generate with: openssl rand -base64 32",
+			len(c.Encryption.ConfigEncryptionKey), minEncryptionKeyLength,
+		)
+	}
+
 	// Validate database configuration
 	if c.Database.URL == "" {
 		return fmt.Errorf("database URL is required")