feat: add TCO API endpoint (refs #15)

- Add GET /api/vehicles/:id/tco route
- Add getTCO controller method with error handling
- Returns 200 with TCO data, 404 for not found, 403 for unauthorized

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

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

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

@@ -166,20 +166,20 @@ export class VehiclesController {
     try {
       const userId = (request as any).user.sub;
       const { id } = request.params;
-      
+
       await this.vehiclesService.deleteVehicle(id, userId);
-      
+
       return reply.code(204).send();
     } catch (error: any) {
       logger.error('Error deleting vehicle', { error, vehicleId: request.params.id, userId: (request as any).user?.sub });
-      
+
       if (error.message === 'Vehicle not found' || error.message === 'Unauthorized') {
         return reply.code(404).send({
           error: 'Not Found',
           message: 'Vehicle not found'
         });
       }
-      
+
       return reply.code(500).send({
         error: 'Internal server error',
         message: 'Failed to delete vehicle'
@@ -187,6 +187,37 @@ export class VehiclesController {
     }
   }
 
+  async getTCO(request: FastifyRequest<{ Params: VehicleParams }>, reply: FastifyReply) {
+    try {
+      const userId = (request as any).user.sub;
+      const { id } = request.params;
+
+      const tco = await this.vehiclesService.getTCO(id, userId);
+      return reply.code(200).send(tco);
+    } catch (error: any) {
+      logger.error('Error getting vehicle TCO', { error, vehicleId: request.params.id, userId: (request as any).user?.sub });
+
+      if (error.statusCode === 404 || error.message === 'Vehicle not found') {
+        return reply.code(404).send({
+          error: 'Not Found',
+          message: 'Vehicle not found'
+        });
+      }
+
+      if (error.statusCode === 403 || error.message === 'Unauthorized') {
+        return reply.code(403).send({
+          error: 'Forbidden',
+          message: 'Not authorized to access this vehicle'
+        });
+      }
+
+      return reply.code(500).send({
+        error: 'Internal server error',
+        message: 'Failed to calculate TCO'
+      });
+    }
+  }
+
   async getDropdownMakes(request: FastifyRequest<{ Querystring: { year: number } }>, reply: FastifyReply) {
     try {
       const { year } = request.query;