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>

services/reader/app/models.py

@@ -68,6 +68,7 @@ class Entry(Base):
     published_at = Column(DateTime)
     status = Column(String(10), default="unread")
     starred = Column(Boolean, default=False)
+    thumbnail = Column(Text)
     reading_time = Column(Integer, default=1)
     created_at = Column(DateTime, default=datetime.utcnow)