feat: phase 4 admin CMS — dashboard, editor, media, CSRF

Head Hen CMS end-to-end: dashboard lists all posts (drafts + published),
Markdown editor with live preview + drag-drop image upload, Pillow media
pipeline re-encoding every upload to JPEG, post CRUD + publish toggle +
hard delete, About page edit, and double-submit CSRF cookie enforced on
every admin mutating endpoint (Phase 3's TODO markers resolved).

Slug auto-generated on create and server-locked once a post has been
published. Unpublish preserves `published_at` so re-publish keeps
original date ordering. Every admin write invalidates the read-side
Post/Page TTL caches and records an `auth_events` audit row.

CSRF middleware is narrow by design — issues/refreshes the `cb_csrf`
cookie only on `GET /admin*`, and mutating endpoints opt in via
`require_csrf_form` or `require_csrf_header` Depends. Public routes,
healthz, and pre-auth login stay untouched.

64 new tests cover slugs, CSRF, media, admin posts/pages services, and
end-to-end CMS routes. Tests never mock the DB — real temp SQLite files
per the CLAUDE.md mandate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -26,11 +26,14 @@ env/
 *.db-shm
 
 # Runtime data (DB + media uploads live here in prod).
-# Ignore everything inside data/ except the .gitkeep marker; negating a
+# Ignore everything inside data/ except the .gitkeep markers; negating a
 # path whose *directory* is ignored does not work, so we ignore the
 # contents with a trailing glob instead.
 data/*
 !data/.gitkeep
+!data/media/
+data/media/*
+!data/media/.gitkeep
 
 # Editors / IDE
 .vscode/