docs(cryptify): document /metrics endpoint and X-Cryptify-Source header

[dobby-coder]

Documents Cryptify's Prometheus /metrics endpoint and the new X-Cryptify-Source channel-tagging header in docs/repos/cryptify.md. All identifiers verified against the cryptify#102 / #165 merge commits.

Changes

• Add metrics_scan_interval_secs row to the configuration table (default 60).
• Repin the src/config.rs source link to the #165 merge commit so the line range covers the new field.
• Add a GET /metrics bullet to the API surface list for parity with docs/guide/architecture.md and docs/repos/postguard.md.
• Add a ## Metrics section with the five Prometheus series, the storage-scan cadence, and the six pre-seeded channels (website, staging-website, outlook, thunderbird, api, unknown) from cryptify#165, including the PromQL increase() rationale.
• Add a ### Channel detection sub-section covering the X-Cryptify-Source -> bearer/api-key -> Origin -> User-Agent -> unknown priority chain, the [a-z0-9_-] / 32-char sanitization, and the client-side header settings from postguard-website#228, postguard-outlook-addon#96, and postguard-tb-addon#121.

Reviewer quickstart

git fetch origin && git checkout docs/cryptify-metrics-endpoint && npm install && npx vitepress dev docs

Open http://localhost:5173/repos/cryptify and look for the new Metrics section between the existing API and Development sections.

Verification

• npx vitepress build docs succeeds in ~6.8 s.
• Built dist/repos/cryptify.html contains the five metric names, the header name, the new config field, and GET /metrics.

Closes #103