feat: thumbnail extraction for Reader — fixes all clients

Server-side (dashboard + iOS + any client):
- Added thumbnail column to reader_entries
- Worker extracts from media:thumbnail, media:content, enclosures, HTML img
- API returns thumbnail in EntryOut with & decoding
- Backfilled 260 existing entries

iOS:
- Prefers API thumbnail, falls back to client-side extraction
- Decodes HTML entities in URLs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/Features/Reader/Models/ReaderModels.swift

@@ -18,6 +18,7 @@ struct ReaderEntry: Codable, Identifiable, Hashable {
     let status: String
     let starred: Bool
     let readingTime: Int
+    let thumbnail: String?
     let feed: ReaderFeedRef?
 
     var isRead: Bool { status == "read" }
@@ -49,7 +50,12 @@ struct ReaderEntry: Codable, Identifiable, Hashable {
     }
 
     var thumbnailURL: URL? {
-        ReaderEntry.extractThumbnail(from: content ?? fullContent ?? "")
+        // Prefer server-provided thumbnail
+        if let thumb = thumbnail, !thumb.isEmpty, let url = URL(string: thumb) {
+            return url
+        }
+        // Fallback: extract from content
+        return ReaderEntry.extractThumbnail(from: content ?? fullContent ?? "")
     }
 
     private static let imgRegex = try! NSRegularExpression(
@@ -68,6 +74,9 @@ struct ReaderEntry: Codable, Identifiable, Hashable {
             return nil
         }
         let src = String(searchRange[srcRange])
+            .replacingOccurrences(of: "&", with: "&")
+            .replacingOccurrences(of: "&lt;", with: "<")
+            .replacingOccurrences(of: "&gt;", with: ">")
         return URL(string: src)
     }