Settings, PLAN: video previews follow the Transcode device

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

PLAN.md

@@ -852,6 +852,16 @@ something long in one, switch away and back, refresh — it's all still there.
   - 360 files: the look-around view, zoom 8°–300° (ordinary lens into stereographic, the
     "little planet"), drag/pinch/wheel, phone motion. Full screen is the element's, or the
     whole window on an iPhone.
+  - **Video previews follow the setting too** (same day, asked for as "it should use the
+    same video generation device"). On the device's own machine the hub says which device
+    by header (`X-Hamlab-Device`); for a file elsewhere the hub asks the file's machine's
+    cache first (`?cached=1`), else has the device's machine make it from the hub's one-file
+    URL (`POST /api/files/thumb-from`) and keeps it in the file's machine's cache (`PUT
+    /api/files/thumb`, refused if the file changed since its listing), so each is made once.
+    A failure is kept as a failure. Unreachable → made where the file is, as before. Images
+    and audio covers stay on the file's machine's CPU (a JPEG is cheaper there than a GPU
+    round trip). Measured: a `hamtv` video previewed on grid's A380 in 0.22 s, then 2 ms
+    from hamtv's cache. The previews button's tooltip names the device.
   - Measured (`web/scripts/e2e-player.mjs`, Chromium): an iPhone HLG clip on `core` made on
     grid's A380 — 3.2 s to play, a seek 1.0 s, a quality change 1.5 s; the 8K 360 Disney day on
     grid — 3.1 s, 2160s on Auto, a seek 4.9 s (10 s source keyframes).