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

lib/pinchflat_web/router.ex

@@ -52,6 +52,8 @@ defmodule PinchflatWeb.Router do
     resources "/search", Searches.SearchController, only: [:show], singleton: true
 
     resources "/settings", Settings.SettingController, only: [:show, :update], singleton: true
+    post "/settings/generate_api_token", Settings.SettingController, :generate_api_token
+    post "/settings/revoke_api_token", Settings.SettingController, :revoke_api_token
     get "/app_info", Settings.SettingController, :app_info
     get "/download_logs", Settings.SettingController, :download_logs