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/api/entries.py

@@ -40,6 +40,7 @@ class EntryOut(BaseModel):
     status: str = "unread"
     starred: bool = False
     reading_time: int = 1
+    thumbnail: str | None = None
     feed: FeedRef | None = None
 
     class Config:
@@ -49,6 +50,10 @@ class EntryOut(BaseModel):
     def from_entry(cls, entry: Entry) -> "EntryOut":
         # Use full_content if available, otherwise RSS content
         best_content = entry.full_content if entry.full_content else entry.content
+        # Extract thumbnail from stored field, or from content
+        thumb = entry.thumbnail
+        if not thumb:
+            thumb = cls._extract_thumbnail(entry.content or entry.full_content or "")
         return cls(
             id=entry.id,
             title=entry.title,
@@ -60,9 +65,25 @@ class EntryOut(BaseModel):
             status=entry.status,
             starred=entry.starred,
             reading_time=entry.reading_time,
+            thumbnail=thumb,
             feed=FeedRef(id=entry.feed.id, title=entry.feed.title) if entry.feed else None,
         )
 
+    @staticmethod
+    def _extract_thumbnail(html: str) -> str | None:
+        """Extract first image URL from HTML content."""
+        if not html:
+            return None
+        import re
+        match = re.search(r'<img[^>]+src=["\']([^"\']+)["\']', html[:3000], re.IGNORECASE)
+        if match:
+            url = match.group(1).replace("&amp;", "&")
+            # Skip tiny tracking pixels and icons
+            if any(skip in url.lower() for skip in ["1x1", "pixel", "tracking", "spacer"]):
+                return None
+            return url
+        return None
+
 
 class EntryListOut(BaseModel):
     total: int