Task 8 of the slice plan. Saves clipboard PNG/JPEG bytes under
{profileDir}/media/{noteId}/{uuid}.{ext} with mkdir -p semantics,
returns relPath/mime/bytes for the repository row, supports
deleteNoteDirectory (used by note delete + GC) and listNoteDirs
(used by media GC to find orphans).
Verification: `npx vitest run tests/unit/MediaStore.test.ts`
4 passed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
