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>

docker-compose.gpu.yml

@@ -0,0 +1,24 @@
+# Overlay for hosts with a VA-API-capable GPU passed through (Intel
+# QSV, AMD VCN/VCE, any VA-API driver). PhotoPrism's :latest image
+# ships VA-API-enabled ffmpeg; this file just wires the device + group
+# membership + encoder selection. Layered in by the deploy script on
+# hosts where /dev/dri/renderD128 exists.
+#
+# Usage:
+#   docker compose -f docker-compose.yml -f docker-compose.gpu.yml up -d
+
+services:
+  photoprism:
+    devices:
+      - /dev/dri/renderD128:/dev/dri/renderD128
+      - /dev/dri/card0:/dev/dri/card0
+    # Match host GIDs (render=992, video=44 on Debian). PhotoPrism's
+    # container user (PP_UID:PP_GID, typically 33:10000) is not in
+    # these groups by default; group_add grants access to the device
+    # nodes without changing the primary user.
+    group_add:
+      - "992"
+      - "44"
+    environment:
+      PHOTOPRISM_FFMPEG_ENCODER: ${PP_FFMPEG_ENCODER:-vaapi}
+      PHOTOPRISM_FFMPEG_BITRATE: ${PP_FFMPEG_BITRATE:-32}