Plex attaches TMDB GUIDs that identify the individual episode, not the parent series, so using them as `tmdbId` caused incorrect series lookups. Now only movie scrobbles populate `tmdbId` from the TMDB GUID; episode scrobbles rely on IMDB/TVDB IDs (and the title) to resolve the show via `resolveShowTmdbId`.
- Extract IMDB/TMDB/TVDB prefix constants to avoid magic strings
- Add `parsePlexPayload` test asserting `tmdbId` is `undefined` for episode events
- Add `processWebhook` integration test verifying `resolveShowTmdbId` is called without a TMDB ID for Plex episodes
- Hoist `makePlexForm` helper to module scope so it's available across all describe blocks