feat: parse FFmpeg progress for structured download status

[cursor]

Problem

StreamDL has no structured download progress data. FFmpeg output goes to logs only, so users cannot see speed, percentage, or ETA without parsing debug output manually.

This blocks a proper TUI (#552) and limits observability for long VOD downloads.

Proposed scope

Parse FFmpeg stderr progress lines during live and VOD downloads and expose structured state internally, e.g.:

• Channel / download type (live vs vod)
• Bytes downloaded / duration recorded
• Speed, percentage (when determinable), ETA

Suggested approach

• Parse FFmpeg -progress output or stderr time= / speed= / size= lines in download_stream.go
• Hold active download state in memory (similar pattern to notice buffer / active user tracking)
• Log at DEBUG by default; reserve INFO for tick summary + notices

Out of scope (follow-ups)

• Full bubbletea TUI (feat: TUI for download progress and stream management #552) — this issue is the data layer only
• Push notifications / webhooks

Dependencies / related

• feat: TUI for download progress and stream management #552 — TUI for download progress and stream management
• Notice buffer (merged feat: tick-scoped notice buffer flushed after wait message #574) — errors/warnings already flushed per tick; progress is complementary

Acceptance criteria

• Active downloads expose parseable progress while FFmpeg is running
• Unit tests for progress line parsing (sample FFmpeg stderr fixtures)
• No regression to existing download completion / move / post_script behaviour