Track media first_seen_at and last_seen_at timestamps

- Add last_seen_at column to Media table (updated on every encounter)
- first_seen_at tracks initial discovery
- last_seen_at tracks most recent sighting
- MediaSource.discovered_at already tracks per-page first link time
- Enables tracking media reuse across the web

database.py

@@ -74,6 +74,7 @@ class Media(Base):
     title = Column(Text)
     score = Column(Integer, default=SCORE_THUMBNAIL)  # Quality score
     first_seen_at = Column(Text, nullable=False)
+    last_seen_at = Column(Text)  # Updated each time media is encountered
     upgraded_at = Column(Text)  # When score was upgraded (for live feed)
     analysis_status = Column(String(20), default='pending')
     analysis_result = Column(Text)  # JSON
@@ -85,6 +86,7 @@ class Media(Base):
         Index('idx_media_analysis', 'analysis_status'),
         Index('idx_media_score', 'score'),
         Index('idx_media_upgraded', 'upgraded_at'),
+        Index('idx_media_last_seen', 'last_seen_at'),
     )
 
 
@@ -235,6 +237,13 @@ class Database:
             except Exception:
                 pass
 
+            # Add last_seen_at column to media table
+            try:
+                await conn.execute(text("ALTER TABLE media ADD COLUMN last_seen_at TEXT"))
+                await conn.execute(text("CREATE INDEX IF NOT EXISTS idx_media_last_seen ON media(last_seen_at)"))
+            except Exception:
+                pass  # Column already exists
+
             # Create FTS5 virtual table (SQLAlchemy doesn't handle virtual tables)
             # Includes description and keywords for better search scoring
             # Check if FTS table needs rebuilding (old schema didn't have description/keywords)
@@ -402,15 +411,19 @@ class Database:
         now = datetime.now(timezone.utc).isoformat()
 
         async with self.session() as session:
-            # Insert or ignore media record using SQLite upsert
+            # Insert or update media record - always update last_seen_at
             media_stmt = sqlite_insert(Media).values(
                 md5_hash=md5_hash,
                 media_type=media_type,
                 mime_type=mime_type,
                 file_size=file_size,
                 score=score,
-                first_seen_at=now
-            ).on_conflict_do_nothing(index_elements=['md5_hash'])
+                first_seen_at=now,
+                last_seen_at=now
+            ).on_conflict_do_update(
+                index_elements=['md5_hash'],
+                set_={'last_seen_at': now}
+            )
             await session.execute(media_stmt)
 
             # Insert or ignore source context