fix(backend): Parse bigint values in heatmap API response

PostgreSQL returns bigint columns as strings. The heatmap API was
returning these raw strings, causing string concatenation instead
of numeric addition in the frontend when summing values.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/multi-state/state-query-service.ts

@@ -823,7 +823,13 @@ export class StateQueryService {
     }
 
     const result = await this.pool.query(query, params);
-    return result.rows;
+    // Parse numeric values from strings (PostgreSQL returns bigint as string)
+    return result.rows.map((row: any) => ({
+      state: row.state,
+      stateName: row.stateName,
+      value: row.value !== null ? parseFloat(row.value) : 0,
+      label: row.label,
+    }));
   }
 
   /**