docs: add book-matching, collections, and OPDS API endpoints

Phase 2 part 6: Split advanced features endpoints
- Book Matching: search_books, link_book (manual and auto-link)
- Collections: INDEX.md linking to COLLECTIONS_API.md
- OPDS: feeds, acquisition, publication (OPDS 1.2 protocol)
- Include format conversion details (EPUB to KEPUB)

docs/api/book-matching/link_book.md

@@ -0,0 +1,92 @@
+# Link Book
+
+Link a device book to a Bookhoard media item. Supports bulk linking.
+
+**Endpoint**: `POST /api/sync/bulk-link-books` or `POST /api/sync/auto-link-books`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Manual Link Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| links | array | Yes | List of book links |
+| links[].unlinked_book_id | string | Yes | Device book UUID |
+| links[].media_item_id | string | Yes | Bookhoard media item UUID |
+| links[].confidence_score | float | No | Match confidence (0-1) |
+
+### Example Manual Link Request
+
+```json
+{
+  "links": [
+    {
+      "unlinked_book_id": "uuid-1",
+      "media_item_id": "uuid-2",
+      "confidence_score": 1.0
+    }
+  ]
+}
+```
+
+## Auto-Link Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| confidence_threshold | float | No | Minimum confidence for auto-link (default: 0.8) |
+| limit | integer | No | Maximum books to auto-link (default: 50) |
+
+### Example Auto-Link Request
+
+```json
+{
+  "confidence_threshold": 0.8,
+  "limit": 50
+}
+```
+
+## Response (200 OK) - Manual Link
+
+```json
+{
+  "results": [
+    {
+      "unlinked_book_id": "uuid-1",
+      "status": "success",
+      "media_item_id": "uuid-2"
+    }
+  ],
+  "total": 1,
+  "successful": 1,
+  "failed": 0
+}
+```
+
+## Response (200 OK) - Auto-Link
+
+```json
+{
+  "auto_linked": 15,
+  "results": [
+    {
+      "unlinked_book_id": "uuid-1",
+      "title": "The Hobbit",
+      "matched_media_item_id": "uuid-2",
+      "confidence": 0.95,
+      "match_method": "sha256_match"
+    }
+  ]
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid link data |
+| 401 | Invalid or expired token |
+| 404 | Media item not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->