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

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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 the Phase 2 follow-up. 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 (Phase 2)
• No Dockerfile / default behavior change

Out of scope

• QSV-capable FFmpeg image (Phase 2)
• First-class FFMPEG_HWACCEL knobs (Phase 3)
• NVIDIA / AMD

Depends on

• None (first slice of [FEATURE] - Allow User to Pass Through Intel QuickSync Device #452)

Blocks

• Phase 2 QSV client image work
• Phase 3 hwaccel presets (practically needs device + capable FFmpeg)

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)