ci(codeql): teach analyzer about ValidateSafeURL SSRF barrier

Closes CodeQL alert #23 (go/request-forgery, Critical) at the
structural level — by telling CodeQL what the runtime code already
does — rather than via per-line `// codeql[...]` suppressions.

Background. internal/service/scep_probe.go:232 calls client.Do(req)
where the request URL is built from operator-supplied input. The
runtime defense is two-layer:

  1. validation.ValidateSafeURL(rawURL) at scep_probe.go:86 rejects
     non-http(s) schemes, empty hosts, literal-IP hosts in reserved
     ranges (loopback, link-local incl. cloud metadata
     169.254.169.254, multicast, broadcast, unspecified, IPv6
     link-local), and DNS names whose A/AAAA resolution returns any
     reserved IP. RFC 1918 is intentionally NOT blocked — see
     internal/validation/ssrf.go:17-21 for the design rationale.

  2. validation.SafeHTTPDialContext on the http.Transport (line 254)
     re-resolves at dial time, applies the same reserved-IP set, and
     pins the dial to a literal non-reserved IP — defeating DNS
     rebinding between validate and dial.

CodeQL's go/request-forgery query is a syntactic taint-tracking rule
with no built-in knowledge of either validator, so it reports the
finding even though the runtime is correctly defended.

The fix. Add a Models-as-Data (MaD) extension at .github/codeql/
declaring ValidateSafeURL as a request-forgery barrier. The barrier
applies to Argument[0] (the URL parameter), which means the analyzer
treats every URL flowing through ValidateSafeURL as sanitized for the
request-forgery taint set. After this lands:

  - Alert #23 dismisses at scep_probe.go:232.
  - The same model applies to the second site of this exact shape —
    webhook notifier's outbound client.Do (internal/connector/
    notifier/webhook/webhook.go) — without per-line annotations.
  - Future code that flows operator URLs through ValidateSafeURL
    inherits the barrier automatically.

This is the structural fix, not a band-aid:

  - Band-aid (rejected): `// codeql[go/request-forgery]` suppression
    on line 232. Suppresses one alert; doesn't teach the analyzer.
    Webhook notifier would need the same comment when its sibling
    rule landing fires.

  - Structural (this change): teach CodeQL via models-as-data, in
    config checked into the repo, that lives next to the workflow
    that uses it. The validators ARE sanitizers in the runtime —
    this PR makes the analyzer's model match reality.

Files:

  - .github/codeql/qlpack.yml — local model pack manifest, declares
    extensionTargets: codeql/go-all: '*'

  - .github/codeql/models/request-forgery-sanitizers.model.yml —
    barrierModel row for validation.ValidateSafeURL Argument[0] /
    request-forgery taint kind / manual provenance

  - .github/codeql/codeql-config.yml — references the local pack +
    keeps security-and-quality query suite scope

  - .github/workflows/codeql.yml — Initialize CodeQL step picks up
    config-file: ./.github/codeql/codeql-config.yml. The existing
    `queries: security-and-quality` line stays so even if the config
    file fails to load, the suite scope is preserved.

  - docs/architecture.md::Input Validation and SSRF Protection —
    extended to name the egress validators (ValidateSafeURL +
    SafeHTTPDialContext) and the call sites (SCEP probe + webhook
    notifier). Closes the docs gap surfaced during the audit; the
    egress threat-model previously lived only in source comments.

Requires CodeQL CLI ≥ 2.25.2 for the barrierModel extensible
predicate (Go MaD support added 2026-04-21). github/codeql-action@v3
ships a recent enough CLI by default; if a future analysis fails
with "unknown extensible predicate barrierModel", the action's CLI
has regressed below 2.25.2 — pin a newer action version rather than
reverting this pack. Documented inline in qlpack.yml.

References:
  - https://codeql.github.com/docs/codeql-language-guides/customizing-library-models-for-go/
  - https://github.blog/changelog/2026-04-21-codeql-now-supports-sanitizers-and-validators-in-models-as-data/

.github/codeql/codeql-config.yml

@@ -0,0 +1,36 @@
+# CodeQL analysis config for certctl.
+#
+# Loaded by .github/workflows/codeql.yml's `Initialize CodeQL` step via
+# `config-file:`. Two responsibilities:
+#
+#  1. Re-declare the query suite the workflow runs (security-and-quality)
+#     so that disabling the action's default suite via `disable-default-
+#     queries: true` doesn't accidentally drop coverage.
+#
+#  2. Load the local model pack at .github/codeql/, which adds project-
+#     specific Models-as-Data extensions (sanitizers, sinks, summaries)
+#     for the standard Go queries. See ./qlpack.yml for the full motivation.
+#
+# Path-ignore is intentionally empty — every path that ships with the
+# repo is in scope. Test files are NOT excluded; if a vulnerability
+# regresses in a test fixture and is later promoted to production, we
+# want CodeQL to catch it on first appearance.
+
+name: certctl-codeql
+
+# Run the same query suite the workflow has been running pre-config-file:
+# security-and-quality (security findings + maintainability/correctness).
+# Listing it here ensures the suite stays in scope even if the action's
+# default behavior shifts.
+queries:
+  - uses: security-and-quality
+
+# Load the local model pack. This is what makes the SSRF sanitizer
+# barrier rows in models/request-forgery-sanitizers.model.yml apply to
+# the standard go/request-forgery query.
+#
+# `${{ }}` is not used here — the path is relative to the config file's
+# directory, not to the repo root, per CodeQL action docs.
+packs:
+  go:
+    - ./