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/db/models/__init__.py

@@ -1,4 +1,35 @@
 from app.db.models.tenant import Tenant
 from app.db.models.user import User
+from app.db.models.vehicle import Vehicle
+from app.db.models.order import Order
+from app.db.models.order_line import OrderLine
+from app.db.models.catalog import (
+    CatalogMarca,
+    CatalogModel,
+    CatalogAnsamblu,
+    CatalogNorma,
+    CatalogPret,
+    CatalogTipDeviz,
+    CatalogTipMotor,
+)
+from app.db.models.invoice import Invoice
+from app.db.models.appointment import Appointment
+from app.db.models.mecanic import Mecanic
 
-__all__ = ["Tenant", "User"]
+__all__ = [
+    "Tenant",
+    "User",
+    "Vehicle",
+    "Order",
+    "OrderLine",
+    "CatalogMarca",
+    "CatalogModel",
+    "CatalogAnsamblu",
+    "CatalogNorma",
+    "CatalogPret",
+    "CatalogTipDeviz",
+    "CatalogTipMotor",
+    "Invoice",
+    "Appointment",
+    "Mecanic",
+]