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>

.env.example

@@ -20,6 +20,11 @@ SECRET_KEY=dev-insecure-change-me
 # --- Database ---------------------------------------------------------------
 DATABASE_URL=sqlite:///data/app.db
 
+# --- Media storage ----------------------------------------------------------
+# Filesystem directory holding admin-uploaded images. Mounted publicly at
+# /media by the app. Relative paths resolve against the process cwd.
+MEDIA_ROOT=data/media
+
 # --- Email (Resend) ---------------------------------------------------------
 RESEND_API_KEY=
 RESEND_FROM=no-reply@chickenbabies.example