docs: add progress, notes, and highlights API endpoints

Phase 2 part 4: Split reading progress and annotation endpoints
- Progress: get_progress, update_progress, delete_progress
- Notes: get_notes, create_note, update_note, delete_note
- Highlights: get_highlights, create_highlight, update_highlight, delete_highlight
- All endpoints support EPUB CFI and percentage locations

docs/api/notes/delete_note.md

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+# Delete Note
+
+Delete a note.
+
+**Endpoint**: `DELETE /api/media-items/notes/{note_id}`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| note_id | string | Yes | Note UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+DELETE /api/media-items/notes/uuid
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (204 No Content)
+
+Note deleted successfully.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | User does not own this note |
+| 404 | Note not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->