feat(backend): sync endpoints + all models + seed + order workflow

- All business models: Vehicle, Order, OrderLine, Invoice, Appointment,
  CatalogMarca/Model/Ansamblu/Norma/Pret/TipDeviz/TipMotor, Mecanic
- Sync endpoints: GET /sync/full, GET /sync/changes?since=, POST /sync/push
  with tenant isolation and last-write-wins conflict resolution
- Order CRUD with state machine: DRAFT -> VALIDAT -> FACTURAT
  Auto-recalculates totals (manopera + materiale)
- Vehicle CRUD: list, create, get, update
- Seed data: 24 marci, 11 ansamble, 6 tipuri deviz, 5 tipuri motoare, 3 preturi
- Alembic migration for all business models
- 13 passing tests (auth + sync + orders)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -7,6 +7,9 @@ from app.auth.router import router as auth_router
 from app.config import settings
 from app.db.base import Base
 from app.db.session import engine
+from app.orders.router import router as orders_router
+from app.sync.router import router as sync_router
+from app.vehicles.router import router as vehicles_router
 
 # Import models so Base.metadata knows about them
 import app.db.models  # noqa: F401
@@ -28,6 +31,9 @@ app.add_middleware(
     allow_credentials=True,
 )
 app.include_router(auth_router, prefix="/api/auth")
+app.include_router(sync_router, prefix="/api/sync")
+app.include_router(orders_router, prefix="/api/orders")
+app.include_router(vehicles_router, prefix="/api/vehicles")
 
 
 @app.get("/api/health")