feat(thumbnails): server-side video thumbnails via ffmpeg

Videos now get a thumbnail generated eagerly server-side on upload, through
the same WebP/blob-hash pipeline as photos — instead of the old browser path
that only ran when the Photos grid first rendered a video tile, re-downloaded
the whole video to seek a frame, and PUT 3 JPEGs back (and produced nothing at
all for HEVC/.mov, which a browser <video> cannot decode).

- New VideoFramePort (application) + FfmpegVideoFrameService / NoopVideoFrameService
  (infrastructure): shell out to the system ffmpeg (no compile-time libav dep),
  extract one representative frame as PNG, bounded by its own semaphore + a
  per-process timeout + kill_on_drop. Noop when ffmpeg is absent/disabled, so
  videos degrade gracefully to no thumbnail.
- ThumbnailRefreshHook.on_file_created routes video/* to
  generate_video_thumbnails_background: stream the (decrypted, reassembled) blob
  to a size- and time-bounded temp file on the data volume, extract a frame, and
  reuse the shared render_and_persist_all_webp helper — so video thumbnails are
  WebP, blob-hash keyed (dedup'd) and content-negotiated, exactly like photos.
- GET thumbnail serves the video's WebP to every client (byte-sniffed
  Content-Type); a genuine miss returns 204.
- Config: OXICLOUD_ENABLE_VIDEO_THUMBNAILS (default true, needs ffmpeg detected
  at startup) + OXICLOUD_FFMPEG_PATH / _CONCURRENCY / _TIMEOUT_SECS / _MAX_MB.
- Dockerfile installs ffmpeg in the runtime image.
- Frontend: drop the client-side generateVideoThumb/frameFromVideo re-download
  path; the server is now the source of truth.

Benchmark (examples/bench_video_thumbnails.rs, needs ffmpeg): 4/4 codecs incl.
HEVC/.mov produce a thumbnail server-side (was 0% for HEVC); ~50-70 ms/frame in
the background; ~3.9 KB preview WebP; up to ~23x less per-first-view transfer on
the test corpus (far more on real multi-MB clips). Methodology in
benches/VIDEO-THUMB.md.

Hardening from an adversarial review: video render holds the decode_semaphore
like the image path; the ffmpeg scale filter bounds both dimensions; the blob
stream has a timeout; the temp file lives on the data volume; the size cap uses
saturating_mul.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -67,9 +67,11 @@ LABEL org.opencontainers.image.title="OxiCloud" \
 
 # Install only necessary runtime dependencies and update packages
 # su-exec is needed by the entrypoint to drop privileges after fixing volume permissions.
+# ffmpeg powers server-side video thumbnail extraction (one frame → WebP pipeline);
+# without it videos simply have no thumbnail (OXICLOUD_ENABLE_VIDEO_THUMBNAILS).
 # No libpq: the pure-Rust sqlx postgres driver never links it.
 RUN apk --no-cache upgrade && \
-    apk add --no-cache libgcc ca-certificates tzdata su-exec && \
+    apk add --no-cache libgcc ca-certificates tzdata su-exec ffmpeg && \
     addgroup -g 1001 -S oxicloud && \
     adduser -u 1001 -S oxicloud -G oxicloud