feat(podcast): add podcast feature with UX improvements - #5420

Backend
- Add podcast data model (PodcastChannel, PodcastEpisode) with migrations
- Implement Subsonic API endpoints: getPodcasts, getNewestPodcasts,
  createPodcastChannel, refreshPodcasts, deletePodcastChannel,
  deletePodcastEpisode, downloadPodcastEpisode, getPodcastEpisode
- Add native REST API endpoints: GET/DELETE /api/podcast,
  GET /api/podcast/preview (feed info without creating channel)
- Inject events.Broker into podcast service for SSE support
- Emit PodcastEpisodeProgress SSE events during download (every 512 KB)
  and on completion/error with status field
- Use HTTP Content-Length as fallback when RSS feed omits enclosure size
- Write ID3 tags (title, album, genre=Podcast) to downloaded files via
  ffmpeg so the library scanner reads correct metadata
- Set MediaFile fields (Title, Album, AlbumID=channelId, AlbumArtist,
  Genre) on episode registration
- Add duplicate URL check in AddChannel
- Add ExistsByURL to PodcastChannelRepository

Frontend - Podcast list
- Add grid/list view toggle (Redux podcastViewReducer) matching album list
- New PodcastGridView component with responsive column count (2-6 cols)
- Cover image 100px in table view
- Remove Feed URL column; add inline copy-to-clipboard button

Frontend - Podcast show (episode list)
- Real-time download progress (%) in Status column via SSE, no polling
  - Spinner only before first SSE event; N% once data arrives
  - Size column removed; Downloading badge replaced with progress
- Completed episodes play on row click; separate play button removed
- Play / Shuffle / Play Next / Add to Queue buttons above episode list
  (only shown when completed episodes exist)
- On download completion, reload episodes to obtain streamId for
  immediate playback without page refresh
- Clicking album name in AudioTitle navigates to podcast channel page

Frontend - Podcast create
- Full-width URL input with Fetch Feed Info button and Enter key support
- Preview card (cover image, title, episode count, description) before
  committing channel creation
- Add Channel button appears only after preview; shows already-registered
  message if channel URL exists

Frontend - Playlist
- Album link navigates to podcast channel page for podcast tracks
  (identified by genre=Podcast)
- Artist column shows '-' for podcast tracks with empty artist field

Closes #5420

Signed-off-by: ji-ho lee <search5@gmail.com>

server/nativeapi/native_api.go

@@ -75,6 +75,7 @@ func (api *Router) routes() http.Handler {
 		api.R(r, "/player", model.Player{}, true)
 		api.R(r, "/transcoding", model.Transcoding{}, conf.Server.EnableTranscodingConfig)
 		api.addRadioRoute(r)
+		api.addPodcastRoute(r)
 		api.R(r, "/tag", model.Tag{}, false)
 		api.R(r, "/scrobble", model.Scrobble{}, false)
 		if conf.Server.EnableSharing {