feat: profile support for independent marker sets

Each profile has its own set of timeline markers, so the same video
can be cut with different settings (e.g. landscape vs portrait) without
markers interfering. Profile selector in the top bar, persisted in
QSettings, stored per-row in the DB.

- Add `profile` column to ProcessedDB schema (migrates existing rows
  to 'default')
- Scope find_similar, get_markers, _get_markers_for by profile
- Add get_profiles() for populating the combo dropdown
- Thread profile through _DBWorker, _after_load, _refresh_markers,
  _on_clip_done, dropEvent, _on_open_files
- Editable profile QComboBox in top bar, refreshed after each export
- 5 new tests for profile isolation and backward compatibility (54 total)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -31,6 +31,7 @@ All clips are exactly 8 seconds — the standard length for foley sound datasets
 - **Playlist** — drag-and-drop or use the Open Files button; right-click to remove items
 - **Playback loop** — plays the exact selection region on loop so you can preview what will be exported
 - **Group operations** — delete or overwrite acts on all sub-clips in a batch, not just one
+- **Profiles** — switch between independent marker sets (e.g. "landscape" vs "portrait") for the same video
 
 ## Keyboard shortcuts
 
@@ -128,7 +129,7 @@ Export history is stored in `~/.8cut.db` (SQLite). The database records filename
 pytest tests/ -v
 ```
 
-49 unit tests covering path builders, ffmpeg command generation, time formatting, database operations, group queries, and annotation handling.
+54 unit tests covering path builders, ffmpeg command generation, time formatting, database operations, group queries, profile isolation, and annotation handling.
 
 ## License