feat(compose): opt-in GPU overlay for VA-API ffmpeg accel

Layer docker-compose.gpu.yml to mount /dev/dri/{card0,renderD128}
into pp-app, add it to render (992) + video (44) groups, and set
PHOTOPRISM_FFMPEG_ENCODER=vaapi. Hosts without a VA-API device just
skip the overlay (`-f docker-compose.yml -f docker-compose.gpu.yml`
becomes opt-in per deploy).

Drops video transcode + thumbnail generation from CPU to the iGPU
where present — large win for HEVC libraries. README documents the
flag; default behavior on the base compose is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -115,6 +115,7 @@ Full instructions in [`sidecar/README.md`](sidecar/README.md#dev-iteration-loop-
 .
 ├── docker-compose.yml         base stack: mariadb + photoprism + sidecar
 ├── docker-compose.podman.yml  rootless-podman overlay (keep-id mapping)
+├── docker-compose.gpu.yml     opt-in VA-API GPU passthrough overlay
 ├── .env.example               required env vars (copy to .env)
 ├── mariadb/init/              first-boot SQL: creates mule_sidecar DB + user
 ├── pp/                        PhotoPrism bind-mounted state (storage, import)
@@ -122,4 +123,18 @@ Full instructions in [`sidecar/README.md`](sidecar/README.md#dev-iteration-loop-
 └── web/                       SvelteKit frontend
 ```
 
+## GPU video acceleration (optional)
+
+Hosts with a VA-API-capable GPU (Intel iGPU, AMD APU, etc.) can layer
+[`docker-compose.gpu.yml`](docker-compose.gpu.yml) to hand `/dev/dri/*`
+to PhotoPrism and switch ffmpeg to hardware encode/decode — a large
+perf win for video thumbnails and HEVC→H.264 transcodes:
+
+```bash
+docker compose -f docker-compose.yml -f docker-compose.gpu.yml up -d
+```
+
+Set `PP_FFMPEG_ENCODER=vaapi` in `.env` (default for the overlay). Verify
+with `docker exec pp-app photoprism show config | grep -i ffmpeg`.
+
 [pp]: https://photoprism.app/