Fix DB init crash: wrap bulk migration in explicit transaction

The COMMIT without BEGIN failed in autocommit mode, crashing ProjectDB
init and leaving _shared_db as None ("Database not initialized").

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db.py

@@ -74,6 +74,8 @@ class ProjectDB:
         """One-time bulk migration: merge separate lora strength keys in all stored sequences."""
         rows = self.conn.execute("SELECT id, data FROM sequences").fetchall()
         updated = 0
+        self.conn.execute("BEGIN")
+        try:
             for row in rows:
                 data = json.loads(row["data"])
                 original = row["data"]
@@ -85,8 +87,11 @@ class ProjectDB:
                         (new_json, row["id"]),
                     )
                     updated += 1
-        if updated:
             self.conn.execute("COMMIT")
+        except Exception:
+            self.conn.execute("ROLLBACK")
+            raise
+        if updated:
             logger.info("Migrated lora keys in %d/%d sequences", updated, len(rows))
 
     def close(self):