Fix migration to use IF NOT EXISTS and Postgres-compatible DEFAULT

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/db.py

@@ -16,7 +16,7 @@ def run_migrations():
         try:
             conn.execute(
                 text(
-                    "ALTER TABLE transaction ADD COLUMN deferred_to_next_cycle BOOLEAN NOT NULL DEFAULT 0"
+                    "ALTER TABLE transaction ADD COLUMN IF NOT EXISTS deferred_to_next_cycle BOOLEAN NOT NULL DEFAULT false"
                 )
             )
             conn.commit()