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/cmd/daemon.go

@@ -243,7 +243,13 @@ func runDaemonStart() error {
 	// Wire: sync receives new tasks → submit to manager or handle stream
 	d.OnTasksClaimed = func(tasks []agent.Task) {
 		for _, t := range tasks {
-			if t.Mode == "stream" {
+			if t.Mode == "seed_file" {
+				// Browser asked us to wrap an arbitrary on-disk file as
+				// a single-file torrent + seed it via WebRTC. Runs in
+				// its own goroutine so a slow / failing seed can't
+				// stall the rest of the claim batch.
+				go handleSeedFileTask(t, torrentDl, agentClient)
+			} else if t.Mode == "stream" {
 				if isStreamingTask(t.ID) {
 					continue
 				}