feat(seed-file): unarr-side handler for browser-on-demand seeding (Fase 4.7.c)

Closes the agent half of in-browser playback for arbitrary files. When
the web app inserts a download_task with mode="seed_file", the daemon
now wraps the on-disk file as a single-file torrent, adds it to the
existing WebRTC-enabled torrent client, and reports the generated
info_hash back so the browser can target /stream/<hash>.

Pieces:

- internal/agent/types.go: Task.FilePath (received from claim) +
  StatusUpdate.InfoHash (sent back). Both serialise compatibly with
  the matching Zod schemas in the Next.js sync route.

- internal/engine/seed_file.go: SeedFile(client, filePath, trackers)
  builds the metainfo via metainfo.Info.BuildFromFilePath +
  bencode.Marshal, then AddTorrent + DownloadAll() so anacrolix
  hashes the file and flips pieces to "have" as it goes. The
  libtorrent piece-size ladder is mirrored from wstracker-probe so
  generated torrents are interoperable with mainstream clients.
  SeedFileOnDownloader is the daemon-facing convenience wrapper —
  bails loud when [downloads.webrtc].enabled = false instead of
  silently producing a torrent no browser can find.

- internal/cmd/seed_file_handler.go: handleSeedFileTask invoked from
  the existing OnTasksClaimed dispatcher in daemon.go for mode=
  seed_file. Validates filePath, calls the engine helper, and pushes
  the resulting info_hash via Client.ReportStatus. Failures (missing
  file, WebRTC disabled, ffmpeg-style oddities) report status="failed"
  + errorMessage so the browser's WatchInBrowserButton can show the
  reason instead of timing out at 60 s.

- internal/cmd/daemon.go: dispatcher learns the seed_file branch in
  the same shape as the existing stream branch.

Tests (6 unit, all green):
- SeedFile rejects missing files + directories.
- SeedFile yields a deterministic info_hash for the same payload across
  fresh clients (web client polls expecting this).
- SeedFileOnDownloader errors when WebRTC is disabled.
- chooseSeedPieceLength matches the ladder breakpoints.
- makeAnnounceList handles nil/empty/partial inputs.

Web side compatible: mode=seed_file is already accepted by the sync
schema; agent.Task.filePath + StatusUpdate.infoHash now propagate
through the existing claim/report endpoints. End-to-end browser ↔
unarr smoke is the next concrete verification step (needs a running
unarr-dev daemon plus library scan + a file with no source torrent).

internal/agent/types.go

@@ -72,6 +72,12 @@ type Task struct {
 	Episode         *int   `json:"episode,omitempty"`        // Episode number
 	ContentYear     *int   `json:"contentYear,omitempty"`    // Year from TMDB (avoids regex on torrent title)
 	CollectionName  string `json:"collectionName,omitempty"` // Collection name (e.g., "Harry Potter Collection")
+
+	// FilePath is the on-disk path of the file the agent is being asked
+	// to operate on. Currently used by mode=seed_file to know which
+	// arbitrary file to wrap as a single-file torrent for browser
+	// streaming; populated by the server from libraryItem.filePath.
+	FilePath string `json:"filePath,omitempty"`
 }
 
 // StreamRequest is a request to stream a completed download from disk.
@@ -95,6 +101,9 @@ type StatusUpdate struct {
 	StreamURL       string `json:"streamUrl,omitempty"`
 	StreamReady     bool   `json:"streamReady,omitempty"`
 	ErrorMessage    string `json:"errorMessage,omitempty"`
+	// mode=seed_file: agent computes the info_hash from the local file
+	// and reports it back so the web player can target /stream/<hash>.
+	InfoHash string `json:"infoHash,omitempty"`
 }
 
 // StatusResponse is returned by the status endpoint.