Refactor: more Post events into commands

[FelipeDefensor]

Following PR #387, several other Post events still match the user-action shape that maps cleanly to commands (single producer, single listener, user-initiated). They were left out of #387 because their listener isn't Player, or because the producer side is multi-source. Tracking here as follow-up.

Loop-related (listener: timeline collection)

• Post.PLAYER_TOGGLE_LOOP — producer: loop toggle button (tilia/ui/player.py:180); listener: TimelineUIs.on_loop_toggle (tilia/ui/timelines/collection/collection.py:459). Suggested: media.loop.toggle.
• Post.PLAYER_CANCEL_LOOP — producers: tilia/app.py:239, tilia/media/player/base.py:138,164,260; listener: TimelineUIs.on_loop_cancel (tilia/ui/timelines/collection/collection.py:458). Suggested: media.loop.cancel. Multi-producer, but command semantics are clean.
• Post.PLAYER_CURRENT_LOOP_CHANGED — producer: TimelineUIs.on_loop_change (tilia/ui/timelines/collection/collection.py:1053); listener: Player.on_loop_changed (tilia/media/player/base.py:70). Suggested: media.loop.change. Programmatic rather than user-initiated, so least clean of the three — could keep as a Post event.

Media load / metadata (single listener, user-initiated)

• Post.APP_MEDIA_LOAD — producers: CLI (tilia/ui/cli/load_media.py:62), Qt UI (tilia/ui/qtui.py:415,429), drag-drop (tilia/ui/timelines/collection/collection.py:398), tests; listener: App.load_media (tilia/app.py:65). Suggested: media.load.
• Post.MEDIA_METADATA_FIELD_ADD — producers: CLI metadata (tilia/ui/cli/metadata/set.py:34); listener: FileManager.on_add_media_metadata_field (tilia/file/file_manager.py:71). Suggested: media.metadata.field.add.
• Post.MEDIA_METADATA_FIELD_SET — producers: metadata window (tilia/ui/windows/metadata.py:125,199), CLI (tilia/ui/cli/metadata/set.py:32,35), tests; listener: FileManager.on_set_media_metadata_field (tilia/file/file_manager.py:70). Suggested: media.metadata.field.set.
• Post.METADATA_UPDATE_FIELDS — producers: metadata window (tilia/ui/windows/metadata.py:180), tests; listener: FileManager.on_update_media_metadata_fields (tilia/file/file_manager.py:72). Suggested: media.metadata.fields.update.

Pattern reminder

Follow the established media.* pattern from #387:

1. Move the listener registration from _setup_requests LISTENS set into _setup_commands as commands.register("media.x", self.on_x).
2. Replace post(Post.X, args) call sites with commands.execute("media.x", args).
3. Remove the Post.X enum entry.
4. Update tests in the same commit.

One commit per refactored event.