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/vehicles/schemas.py

@@ -0,0 +1,21 @@
+from pydantic import BaseModel
+
+
+class CreateVehicleRequest(BaseModel):
+    nr_auto: str
+    marca_id: str | None = None
+    model_id: str | None = None
+    an_fabricatie: int | None = None
+    vin: str | None = None
+    proprietar_nume: str | None = None
+    proprietar_telefon: str | None = None
+
+
+class UpdateVehicleRequest(BaseModel):
+    nr_auto: str | None = None
+    marca_id: str | None = None
+    model_id: str | None = None
+    an_fabricatie: int | None = None
+    vin: str | None = None
+    proprietar_nume: str | None = None
+    proprietar_telefon: str | None = None