feat(plex): add plex integration

[dylfrancis]

Addresses #63

Summary

• Added a PlexClient service implementing Plex's PIN-based OAuth flow (generate PIN, build app.plex.tv auth URL, poll for token, validate token) plus server discovery via plex.tv/api/v2/resources.
• Added Plex Media Server operations: identity/machineIdentifier capture, music library create + location reconcile, library scan, paginated track listing, and regular audio playlist create/replace/delete.
• Wired Plex into the Weekly Flow playlist manager: builds one regular playlist per flow/shared playlist from indexed tracks (Plex has no .nsp smart-playlist equivalent), grouped by the weekly-flow subfolder, with a per-playlist fingerprint cache to skip no-op rebuilds and a non-blocking background catch-up after scans.
• Added Plex settings endpoints (/settings/plex/auth/pin, /auth/check, /resources, /test, /sync), persisted plex integration config (url, token, clientId, machineIdentifier, downloadsPath) with the token encrypted, and protected token/clientId from being wiped by blank values on save.
• Added a "Plex downloads path" override so the library points at the path the Plex container sees when it differs from Aurral's host path.
• Added the Plex Settings UI: OAuth connect/reconnect, auto-populating server dropdown, server URL, downloads-path field, Test connection, and Sync-to-Plex-now.
• Named Plex playlists by the bare flow name (no [A]/[AS] prefix) and clean up any previously-created prefixed playlists.

[dylfrancis]

@lklynet still some testing to be done and quirks i need to figure out, but figured i'd put up the PR as it's mostly done for you to take a look at.