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/feeds.md

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+# OPDS Feeds
+
+Bookhoard provides OPDS 1.2 feeds for device compatibility.
+
+## Get Device Catalog
+
+**Endpoint**: `GET /opds/devices/{deviceId}/catalog`
+**Auth**: Device token required
+
+### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| page | integer | No | Page number (default: 1) |
+| per_page | integer | No | Items per page (default: 50, max: 200) |
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/catalog?page=1&per_page=50
+```
+
+### Response (200 OK - OPDS 1.2 XML)
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom" 
+       xmlns:opds="http://opds-spec.org/2010/"
+       xmlns:dc="http://purl.org/dc/elements/1.1/">
+  <id>urn:uuid:device-id</id>
+  <title>Bookhoard Library</title>
+  <updated>2026-02-01T12:00:00Z</updated>
+  
+  <link rel="self" href="http://localhost:8765/opds/devices/kobo-id/catalog"/>
+  <link rel="search" href="http://localhost:8765/opds/devices/kobo-id/search"/>
+  <link rel="start" href="http://localhost:8765/opds/devices/kobo-id/nav"/>
+  
+  <entry>
+    <id>urn:uuid:bookhoard-uuid-123</id>
+    <dc:title>The Hobbit</dc:title>
+    <dc:creator>J.R.R. Tolkien</dc:creator>
+    <updated>2026-02-01T10:00:00Z</updated>
+    
+    <link href="http://localhost:8765/opds/devices/kobo-id/download/uuid-123" 
+          type="application/epub+zip" 
+          rel="http://opds-spec.org/acquisition/open-access"/>
+    
+    <link href="http://localhost:8765/opds/devices/kobo-id/download/uuid-123?format=kepub" 
+          type="application/vnd.kobo+xml+zip" 
+          rel="alternate"/>
+    
+    <dc:identifier id="bookhoard">uuid-123</dc:identifier>
+    <meta property="bookhoard:sha256">abc123...</meta>
+  </entry>
+</feed>
+```
+
+## Search OPDS Catalog
+
+**Endpoint**: `GET /opds/devices/{deviceId}/search`
+**Auth**: Device token required
+
+### Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| q | string | Yes | Search query |
+
+### Example Request
+
+```http
+GET /opds/devices/kobo-id/search?q=Hobbit
+```
+
+### Response (200 OK - OPDS 1.2 XML with search results)
+
+Returns OPDS feed with matching books.
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->