feat: add vehicle selection and downgrade flow - M5 (refs #55)

backend/src/features/subscriptions/api/subscriptions.controller.ts

@@ -246,4 +246,65 @@ export class SubscriptionsController {
       });
     }
   }
+
+  /**
+   * POST /api/subscriptions/downgrade - Downgrade subscription with vehicle selection
+   */
+  async downgrade(
+    request: FastifyRequest<{
+      Body: {
+        targetTier: 'free' | 'pro';
+        vehicleIdsToKeep: string[];
+      };
+    }>,
+    reply: FastifyReply
+  ): Promise<void> {
+    try {
+      const userId = (request as any).user.sub;
+      const { targetTier, vehicleIdsToKeep } = request.body;
+
+      // Validate inputs
+      if (!targetTier || !vehicleIdsToKeep) {
+        reply.status(400).send({
+          error: 'Missing required fields',
+          message: 'targetTier and vehicleIdsToKeep are required',
+        });
+        return;
+      }
+
+      if (!['free', 'pro'].includes(targetTier)) {
+        reply.status(400).send({
+          error: 'Invalid tier',
+          message: 'targetTier must be "free" or "pro"',
+        });
+        return;
+      }
+
+      if (!Array.isArray(vehicleIdsToKeep)) {
+        reply.status(400).send({
+          error: 'Invalid vehicle selection',
+          message: 'vehicleIdsToKeep must be an array',
+        });
+        return;
+      }
+
+      // Downgrade subscription
+      const updatedSubscription = await this.service.downgradeSubscription(
+        userId,
+        targetTier,
+        vehicleIdsToKeep
+      );
+
+      reply.status(200).send(updatedSubscription);
+    } catch (error: any) {
+      logger.error('Failed to downgrade subscription', {
+        userId: (request as any).user?.sub,
+        error: error.message,
+      });
+      reply.status(500).send({
+        error: 'Failed to downgrade subscription',
+        message: error.message,
+      });
+    }
+  }
 }