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/highlights/create_highlight.md

@@ -0,0 +1,66 @@
+# Create Highlight
+
+Create a new highlight for a media item.
+
+**Endpoint**: `POST /api/media-items/{media_id}/highlights`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| selection_text | string | Yes | Highlighted text |
+| start_position | string | No | Start position (e.g., epubcfi) |
+| end_position | string | No | End position (e.g., epubcfi) |
+| color | string | No | Highlight color (hex, default: "#ffff00") |
+| percentage_start | float | No | Start percentage (0-1) |
+| percentage_end | float | No | End percentage (0-1) |
+
+### Example Request
+
+```json
+{
+  "selection_text": "Highlighted text...",
+  "start_position": "epubcfi(/6/4/2:15)",
+  "end_position": "epubcfi(/6/4/2:20)",
+  "color": "#ffff00",
+  "percentage_start": 0.45,
+  "percentage_end": 0.47
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "id": "uuid",
+  "media_item_id": "uuid",
+  "user_id": "uuid",
+  "selection_text": "Highlighted text...",
+  "start_position": "epubcfi(/6/4/2:15)",
+  "end_position": "epubcfi(/6/4/2:20)",
+  "color": "#ffff00",
+  "percentage_start": 0.45,
+  "percentage_end": 0.47,
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid highlight data |
+| 401 | Invalid or expired token |
+| 404 | Media item not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->