feat(plex): filter the feed by playback class (Direct Play / Direct Stream / Transcode HW/SW)

Adds a movie-scoped "Playback" filter to the Plex feed so a user can isolate items by how
they play — the groundwork for collecting HW-transcodable (or no-transcode) versions.

Four classes derived from the stored `playable` + a new `transcode_hw` facet:
- Direct Play (direct), Direct Stream (remux)
- Transcode · HW  — playable=transcode AND the iGPU can hardware-transcode it
- Transcode · SW  — playable=transcode, software libx264 only

The HW/SW split is precise, not codec-only: a 10-bit/HDR HEVC is HW-decodable yet breaks the
VAAPI nv12 pipeline (the same case plex/stream special-cases), so it must count as SW.

Backend:
- New PlexItem columns (migration 0077): transcode_hw, video_bit_depth, media_probed_at.
  Migration backfills transcode_hw codec-only so the filter works immediately.
- `plex.sync.transcode_hw_capable` is the single classifier; the HW-decodable codec set moves
  to sync as `HW_DECODABLE_CODECS` and plex.stream imports it, so the catalog facet and the
  runtime encoder choice can't disagree by codec.
- Bit-depth backfill (`_probe_media_batch`, wired into sync): probes only HW-decodable transcode
  items' video Stream for bit-depth + HDR and refines a 10-bit/HDR HEVC to software. Bounded
  per run, watermarked by media_probed_at. A codec change resets the watermark (stale probe).
- Filter + self-excluded facet on a shared `playback_class_expr` CASE (movie-leg only — a show
  is an aggregate with no single class). transcode_hw threaded onto the feed/detail/card JSON.

Frontend:
- Multi-select "Playback" Combobox in the sidebar (movie scope, facet-empty drop-out), active
  chips, 4-way corner-dot badge (green/blue/amber/red) + tooltip, and an info-page playback chip
  with a codec · bit-depth subtitle. New `bg-info` (blue) token, i18n HU/EN parity.
- Fix a regression where the card's playback dot lost its hover tooltip: the full-card reveal
  overlay swallowed the cursor; the dot now sits above it (z-20).

Gate: backend full lane + frontend typecheck/lint/knip/prettier/vitest all green. Browser-UAT'd
on localdev (filter narrows to the SW set, facet counts, badge colour + tooltip, info chip).

backend/app/routes/plex/detail.py

@@ -411,6 +411,10 @@ def item_detail(
         "year": it.year,
         "duration_seconds": it.duration_s,
         "playable": it.playable,
+        # Playback-class detail: HW/SW transcode split + the codec/bit-depth the info-page chip shows.
+        "transcode_hw": it.transcode_hw,
+        "codec_video": it.codec_video,
+        "video_bit_depth": it.video_bit_depth,
         "thumb": f"/api/plex/image/{it.rating_key}",
         "art": f"/api/plex/image/{it.rating_key}?variant=art",
         "library": _lib_key(db, it.library_id),