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/opds/acquisition.md

@@ -0,0 +1,72 @@
+# OPDS Acquisition
+
+Download books and list available formats.
+
+## Download Book with Format Conversion
+
+**Endpoint**: `GET /opds/devices/{deviceId}/download/{bookId}`
+**Auth**: Device token required
+
+### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| format | string | No | Book format: `epub` (default), `kepub` |
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/download/uuid-123?format=kepub
+```
+
+### Response (200 OK)
+
+**Headers:**
+- `Content-Type`: `application/epub+zip` or `application/vnd.kobo+xml+zip`
+- `Content-Disposition`: attachment; filename="The Hobbit.epub"
+- `X-Bookhoard-UUID`: uuid-123
+- `X-Bookhoard-SHA256`: abc123... (original hash)
+- `X-Bookhoard-KEPUB-SHA256`: xyz789... (KEPUB hash if format=kepub)
+
+**Body:** Book file binary data
+
+## List Available Formats
+
+**Endpoint**: `GET /opds/devices/{deviceId}/formats/{bookId}`
+**Auth**: Device token required
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/formats/uuid-123
+```
+
+### Response (200 OK)
+
+```json
+{
+  "media_item_id": "uuid-123",
+  "formats": [
+    {
+      "format_type": "epub",
+      "file_path": "/path/to/book.epub",
+      "file_sha256": "abc123...",
+      "file_size_bytes": 2456789,
+      "mime_type": "application/epub+zip",
+      "available": true
+    },
+    {
+      "format_type": "kepub",
+      "file_path": "/cache/book.kepub.epub",
+      "file_sha256": "xyz789...",
+      "file_size_bytes": 2478932,
+      "mime_type": "application/vnd.kobo+xml+zip",
+      "available": true
+    }
+  ]
+}
+```
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->