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).

frontend/src/i18n/locales/en/plex.json

@@ -104,6 +104,13 @@
     "collectionEmpty": "No collections",
     "matchLabel": "Match",
     "studio": "Studio",
+    "playable": "Playback",
+    "playableOpt": {
+      "direct": "Direct Play",
+      "remux": "Direct Stream",
+      "transcode_hw": "Transcode · HW",
+      "transcode_sw": "Transcode · SW"
+    },
     "moreRefine": "{{count}} more — refine with search"
   },
   "count": "{{count}} titles",
@@ -285,8 +292,10 @@
   },
   "playable": {
     "direct": "Plays directly in the browser",
-    "remux": "Needs a light remux (no video re-encode)",
-    "transcode": "Needs transcoding (heavier)"
+    "remux": "Direct Stream — a light remux (no video re-encode)",
+    "transcode": "Needs transcoding (heavier)",
+    "transcode_hw": "Transcoded on the hardware encoder",
+    "transcode_sw": "Transcoded in software (heaviest)"
   },
   "playlistAdd": {
     "manage": "Add to playlist",

frontend/src/i18n/locales/hu/plex.json

@@ -104,6 +104,13 @@
     "collectionEmpty": "Nincs kollekció",
     "matchLabel": "Egyezés",
     "studio": "Stúdió",
+    "playable": "Lejátszás",
+    "playableOpt": {
+      "direct": "Közvetlen lejátszás",
+      "remux": "Direct Stream",
+      "transcode_hw": "Transcode · HW",
+      "transcode_sw": "Transcode · SW"
+    },
     "moreRefine": "még {{count}} — finomíts kereséssel"
   },
   "count": "{{count}} cím",
@@ -285,8 +292,10 @@
   },
   "playable": {
     "direct": "Közvetlenül játszható a böngészőben",
-    "remux": "Könnyű remux kell (nincs videó-újrakódolás)",
-    "transcode": "Transcode kell (nehezebb)"
+    "remux": "Direct Stream — könnyű remux (nincs videó-újrakódolás)",
+    "transcode": "Transcode kell (nehezebb)",
+    "transcode_hw": "Hardveres kódolóval transcode-olva",
+    "transcode_sw": "Szoftveresen transcode-olva (legnehezebb)"
   },
   "playlistAdd": {
     "manage": "Listához adás",