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/test.exs

@@ -12,10 +12,6 @@ config :pinchflat,
 
 config :pinchflat, Oban, testing: :manual
 
-# API token is not set in tests by default — allows tests to run without auth.
-# Individual tests that need auth can set it via Application.put_env.
-config :pinchflat, :api_token, nil
-
 # Configure your database
 #
 # The MIX_TEST_PARTITION environment variable can be used