New POST /api/v1/request endpoint accepts a search query from external
sources (Discord bots, Home Assistant, curl) and triggers the
search-match-download pipeline asynchronously. Returns a request_id
for status polling via GET /api/v1/request/<id>. Optional notify_url
for callback on completion.
Also adds webhook_received trigger type and search_and_download action
type to the automation engine, so users can build custom flows like
"when webhook received → search & download → notify Discord".
Includes info panel in Settings showing endpoint URL and curl example.
Adds a full public REST API at /api/v1/ with 32 endpoints covering library, search, downloads, wishlist, watchlist, playlists, system status, and settings. Includes API key authentication (Bearer token), per-endpoint rate limiting, and consistent JSON response format. API keys can be generated and managed from the Settings page. No changes to existing functionality — the API delegates to the same backend services the web UI uses.
