feat: create ownership_costs backend feature capsule (refs #29)

Milestone 1: Complete backend feature with:
- Migration with CHECK (amount > 0) constraint
- Repository with mapRow() for snake_case -> camelCase
- Service with CRUD and vehicle authorization
- Controller with HTTP handlers
- Routes registered at /api/ownership-costs
- Validation with Zod schemas
- README with endpoint documentation

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

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

backend/src/features/ownership-costs/api/ownership-costs.validation.ts

@@ -0,0 +1,18 @@
+import { z } from 'zod';
+import { CreateOwnershipCostSchema, UpdateOwnershipCostSchema, OwnershipCostTypeSchema } from '../domain/ownership-costs.types';
+
+export const ListQuerySchema = z.object({
+  vehicleId: z.string().uuid().optional(),
+  costType: OwnershipCostTypeSchema.optional(),
+  documentId: z.string().uuid().optional(),
+});
+
+export const IdParamsSchema = z.object({ id: z.string().uuid() });
+
+export const CreateBodySchema = CreateOwnershipCostSchema;
+export const UpdateBodySchema = UpdateOwnershipCostSchema;
+
+export type ListQuery = z.infer<typeof ListQuerySchema>;
+export type IdParams = z.infer<typeof IdParamsSchema>;
+export type CreateBody = z.infer<typeof CreateBodySchema>;
+export type UpdateBody = z.infer<typeof UpdateBodySchema>;