feat: parse FFmpeg progress for structured download status

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 (#552) — this issue is the data layer only
• Push notifications / webhooks

Dependencies / related

• #552 — TUI for download progress and stream management
• Notice buffer (merged #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

Start in download_stream.go and trace how FFmpeg is launched for live and VOD downloads. Define the in-memory active download state and parse the proposed progress fields, then add unit tests using sample FFmpeg stderr fixtures. Done means active downloads expose parseable progress without regressing completion, move, or post_script behavior.