Make API auth mandatory, manage tokens via web UI

BREAKING CHANGE: API authentication is now always required. The
PINCHFLAT_API_TOKEN env var is no longer used. Instead, tokens are
stored in the database and managed via Settings → API Access.

Changes:
- Add api_token column to settings table (migration)
- ApiAuthPlug reads from DB; returns 401 if no token configured
- Add API Access section to Settings page with generate/regenerate/revoke
- Add POST /settings/generate_api_token and /settings/revoke_api_token
- Remove api_token from config.exs and runtime.exs
- Update all API controller tests to set auth token in setup
- Update auth plug tests for mandatory authentication
- 1008 tests pass, zero warnings

config/config.exs

@@ -26,7 +26,6 @@ config :pinchflat,
   basic_auth_username: "",
   basic_auth_password: "",
   expose_feed_endpoints: false,
-  api_token: "",
   file_watcher_poll_interval: 1000,
   timezone: "UTC",
   base_route_path: "/"