Fix cascading InFailedSQLTransactionError in covers forecast

When forecast_rooms_for_date throws inside covers_model, asyncpg aborts the
whole transaction. The exception was being caught with a warning but no rollback,
so every subsequent query in the same request failed. Added db.rollback() in
all three exception handlers (breakfast pickupv2, dinner pickupv2, per-day loop)
and moved public.py's rollback into a finally block so it always fires even
when covers_model returns normally with a corrupted transaction.

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

backend/api/public.py

@@ -384,8 +384,11 @@ async def get_revenue_forecast(
         covers_by_date = {c["date"]: c for c in covers_data.get("data", [])}
     except Exception as e:
         logger.warning(f"Covers forecast failed: {e}")
-        await db.rollback()
         covers_by_date = {}
+    finally:
+        # covers_model catches exceptions internally and may leave the transaction
+        # aborted without raising — always rollback so subsequent queries succeed
+        await db.rollback()
 
     # DOW averages from recent 8 weeks — fallback when covers gives zero for future dates
     dow_result = await db.execute(