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/publication.md

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+# OPDS Publication
+
+OPDS navigation and publication feeds.
+
+## Navigation Feed
+
+**Endpoint**: `GET /opds/devices/{deviceId}/nav`
+**Auth**: Device token required
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/nav
+```
+
+### Response (200 OK - OPDS 1.2 Navigation XML)
+
+Returns OPDS navigation feed with links to:
+- Root catalog
+- Search
+- Collections/shelves
+- Filtered views (by author, series, etc.)
+
+## Publication Feeds by Collection
+
+**Endpoint**: `GET /opds/devices/{deviceId}/collections/{collectionId}`
+**Auth**: Device token required
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/collections/collection-uuid
+```
+
+### Response (200 OK - OPDS 1.2 XML)
+
+Returns OPDS feed with books in the specified collection.
+
+## Publication Feeds by Shelf
+
+**Endpoint**: `GET /opds/devices/{deviceId}/shelves/{shelfName}`
+**Auth**: Device token required
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/shelves/Favorites
+```
+
+### Response (200 OK - OPDS 1.2 XML)
+
+Returns OPDS feed with books on the specified device shelf.
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->