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/runtime.exs

@@ -23,8 +23,7 @@ end
 
 config :pinchflat,
   basic_auth_username: System.get_env("BASIC_AUTH_USERNAME"),
-  basic_auth_password: System.get_env("BASIC_AUTH_PASSWORD"),
-  api_token: System.get_env("PINCHFLAT_API_TOKEN")
+  basic_auth_password: System.get_env("BASIC_AUTH_PASSWORD")
 
 arch_string = to_string(:erlang.system_info(:system_architecture))