Remove unused heartbeat endpoint and storage method

- Removed PUT /offers/:offerId/heartbeat endpoint
- Removed updateOfferLastSeen() from storage interface and implementations
- last_seen column is still in DB but not used for cleanup
- Cleanup only uses expires_at, so heartbeat was non-functional

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/storage/sqlite.ts

@@ -176,16 +176,6 @@ export class SQLiteStorage implements Storage {
     return this.rowToOffer(row);
   }
 
-  async updateOfferLastSeen(offerId: string, lastSeen: number): Promise<void> {
-    const stmt = this.db.prepare(`
-      UPDATE offers
-      SET last_seen = ?
-      WHERE id = ? AND expires_at > ?
-    `);
-
-    stmt.run(lastSeen, offerId, Date.now());
-  }
-
   async deleteOffer(offerId: string, ownerPeerId: string): Promise<boolean> {
     const stmt = this.db.prepare(`
       DELETE FROM offers