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/progress/delete_progress.md

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

docs/api/progress/get_progress.md

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+# Get Reading Progress
+
+Retrieve reading progress for a specific media item.
+
+**Endpoint**: `GET /api/media-items/{media_id}/progress`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/media-items/uuid/progress
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "media_item_id": "uuid",
+  "user_id": "uuid",
+  "current_page": 45,
+  "total_pages": 200,
+  "percentage": 0.225,
+  "character_offset": 15432,
+  "epubcfi": "epubcfi(/6/4/2:15)",
+  "chapter": 3,
+  "chapter_progress": 0.5,
+  "last_read_at": "2026-01-31T10:00:00Z",
+  "format_group": "reflowable",
+  "viewport_y": 0.12,
+  "zoom_level": 1.0
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 404 | Media item not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/api/progress/update_progress.md

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+# Update Reading Progress
+
+Update reading progress for a media item. This will sync across all devices via WebSocket.
+
+**Endpoint**: `PUT /api/media-items/{media_id}/progress`
+**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 |
+|--------|------|-----------|-------------|
+| source | string | Yes | Progress source (e.g., "web", "koreader", "kobo") |
+| location | object | Yes | Location information |
+| location.percentage | float | No | Progress percentage (0-1) |
+| location.epubcfi | string | No | EPUB CFI location |
+| location.character | integer | No | Character offset |
+| location.chapter | integer | No | Chapter number |
+| location.page | integer | No | Current page |
+| location.total_pages | integer | No | Total pages |
+| device_metadata | object | No | Device metadata |
+| device_metadata.device_type | string | No | Device type |
+| device_metadata.user_agent | string | No | User agent string |
+
+### Example Request
+
+```json
+{
+  "source": "web",
+  "location": {
+    "percentage": 0.45678,
+    "epubcfi": "epubcfi(/6/4/2:15)",
+    "character": 15432,
+    "chapter": 3,
+    "page": 89,
+    "total_pages": 200
+  },
+  "device_metadata": {
+    "device_type": "web",
+    "user_agent": "Mozilla/5.0..."
+  }
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "sync_status": "success",
+  "progress_updated": true,
+  "devices_notified": ["device-1", "device-2"],
+  "broadcast": true
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid location data |
+| 401 | Invalid or expired token |
+| 404 | Media item not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->