docs(reference): v1.2.0 reference updates (registry-mirrors env, cache-ttl default, scanner errors)

[brandonrc]

Summary

Three reference-doc updates needed for v1.2.0. All three changes are on main only and not backported to release/1.1.x, so they should land in the docs flip alongside the v1.2.0 release.

Items

1. New env var: AK_DEFAULT_DOCKER_MIRROR_REPO (artifact-keeper#944)

Adds Docker-daemon registry-mirrors fallback (env-configured) when repo_key misses. Currently 0 references in site docs.

• src/content/docs/docs/reference/environment.mdx: add row in environment-variables reference.
• src/content/docs/docs/deployment/docker.mdx: add a small section or example showing how to set it for a Docker daemon registry-mirrors use case.
• src/content/docs/docs/guides/docker.mdx: add a brief mention if relevant for end-user pull flows.

2. CACHE_TTL_SECONDS default change 3600 to 86400 (artifact-keeper#911 / #932)

Backend now uses 24h default to match the proxy_service constant. The reference page is stale.

• src/content/docs/docs/reference/environment.mdx:279: change documented default from 3600 to 86400.
• Verify any deployment guide that references the cache TTL default is consistent.

3. Scanner API error semantics (artifact-keeper#933, #941)

• 503 Service Unavailable when scanner is not configured (was 500).

• 404 Not Found for unknown scan_id (was 200 with empty body).

• src/content/docs/docs/security/scanning.mdx: add an "API error responses" section listing these status codes and when they fire.

Acceptance criteria

• All three items reflected in published docs at v1.2.0 cut.
• No stale 3600 default left in reference docs.
• New env var indexed in environment.mdx and findable via search.

Related

• artifact-keeper#944 OCI registry-mirrors fallback
• artifact-keeper#932 / #911 cache-ttl default unification
• artifact-keeper#933 scanner 503
• artifact-keeper#941 scan_id 404