feat(qsv): first-class FFmpeg hwaccel / codec presets

[cursor]

Goal

Add first-class StreamDL configuration for hardware-accelerated encode/decode (Intel QSV / VAAPI) instead of relying only on raw FFMPEG_EXTRA_* strings.

Context

Parent epic: #452
Depends on Phase 1 (device access) and Phase 2 (QSV-capable FFmpeg image).

Scope

• Env/flags such as FFMPEG_HWACCEL=none|qsv|vaapi and video codec selection (copy / software / h264_qsv / h264_vaapi, etc.)
• Safe defaults preserving current behavior
• Soft failure: clear notice + fallback to copy/software when hwaccel init fails (configurable)
• Ensure download progress (feat: parse FFmpeg progress for structured download status #577) remains correct under hwaccel
• Keep FFMPEG_EXTRA_INPUT_OPTS / FFMPEG_EXTRA_OUTPUT_OPTS as escape hatches

Out of scope

• Automatic GPU vendor discovery
• NVIDIA / AMD (separate issues)
• Forcing re-encode when stream copy is sufficient

Depends on

• Phase 1: /dev/dri passthrough docs/compose
• Phase 2: optional QSV/VAAPI-capable client image

Acceptance

• Documented presets for QSV and/or VAAPI on the QSV client image
• Default (no hwaccel) behavior unchanged
• Missing device/hwaccel produces a clear notice and configurable fallback
• Live + VOD paths covered; progress summary still works