feat(qsv): document and expose Intel /dev/dri device passthrough

Goal

Opt-in Docker Compose / docs so operators can pass /dev/dri (Intel Quick Sync / VAAPI render node) into the StreamDL client container.

Context

Parent epic: #452

This is Phase 1 of Quick Sync support. Device passthrough alone does not enable QSV with the current mwader/static-ffmpeg client image — that lands in #608. This issue only makes the ops surface honest and ready.

Scope

• Extend docker-compose.yml.example with a commented, opt-in devices: (+ optional group_add for video/render) on the client service
• README: host prerequisites (/dev/dri/renderD*), finding GIDs, PUID/PGID interaction, and clear note that a QSV-capable FFmpeg is still required (#608)
• No Dockerfile / default behavior change

Out of scope

• QSV-capable FFmpeg image (#608)
• First-class FFMPEG_HWACCEL knobs (#609)
• NVIDIA / AMD

Depends on

• None (first slice of #452)

Blocks

• #608
• #609

Acceptance

• Compose example documents opt-in /dev/dri passthrough
• README covers host setup and limitations with static FFmpeg
• Default install path unchanged (no required devices)

Start by reading docker-compose.yml.example and the README sections covering container setup and hardware requirements. Document an opt-in /dev/dri passthrough with optional video/render group access, host prerequisites, GID and PUID/PGID considerations, and the static FFmpeg limitation. Done means the default install remains unchanged and all three acceptance points are covered.