feat: add document-vehicle API endpoints and context-aware delete (refs #31)

Updates documents backend service and API to support multi-vehicle insurance documents:
- Service: createDocument/updateDocument validate and handle sharedVehicleIds for insurance docs
- Service: addVehicleToDocument validates ownership and adds vehicles to shared array
- Service: removeVehicleFromDocument with context-aware delete logic:
  - Shared vehicle only: remove from array
  - Primary with no shared: soft delete document
  - Primary with shared: promote first shared to primary
- Service: getDocumentsByVehicle returns all docs for a vehicle (primary or shared)
- Controller: Added handlers for listByVehicle, addVehicle, removeVehicle with proper error handling
- Routes: Added POST/DELETE /documents/:id/vehicles/:vehicleId and GET /documents/by-vehicle/:vehicleId
- Validation: Added DocumentVehicleParamsSchema for vehicle management routes

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

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

backend/src/features/documents/api/documents.validation.ts

@@ -9,6 +9,10 @@ export const ListQuerySchema = z.object({
 
 export const IdParamsSchema = z.object({ id: z.string().uuid() });
 export const VehicleParamsSchema = z.object({ vehicleId: z.string().uuid() });
+export const DocumentVehicleParamsSchema = z.object({
+  id: z.string().uuid(),
+  vehicleId: z.string().uuid()
+});
 
 export const CreateBodySchema = CreateDocumentBodySchema;
 export const UpdateBodySchema = UpdateDocumentBodySchema;
@@ -16,6 +20,7 @@ export const UpdateBodySchema = UpdateDocumentBodySchema;
 export type ListQuery = z.infer<typeof ListQuerySchema>;
 export type IdParams = z.infer<typeof IdParamsSchema>;
 export type VehicleParams = z.infer<typeof VehicleParamsSchema>;
+export type DocumentVehicleParams = z.infer<typeof DocumentVehicleParamsSchema>;
 export type CreateBody = z.infer<typeof CreateBodySchema>;
 export type UpdateBody = z.infer<typeof UpdateBodySchema>;