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/tests/conftest.py

@@ -1,5 +1,6 @@
 import pytest
 import pytest_asyncio
+from httpx import ASGITransport, AsyncClient
 from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 
 from app.db.base import Base
@@ -22,3 +23,40 @@ async def setup_test_db():
     yield
     app.dependency_overrides.clear()
     await engine.dispose()
+
+
+@pytest_asyncio.fixture
+async def client():
+    async with AsyncClient(
+        transport=ASGITransport(app=app), base_url="http://test"
+    ) as c:
+        yield c
+
+
+@pytest_asyncio.fixture
+async def auth_headers(client):
+    r = await client.post(
+        "/api/auth/register",
+        json={
+            "email": "test@service.ro",
+            "password": "testpass123",
+            "tenant_name": "Test Service",
+            "telefon": "0722000000",
+        },
+    )
+    token = r.json()["access_token"]
+    return {"Authorization": f"Bearer {token}"}
+
+
+@pytest_asyncio.fixture
+async def tenant_id(client):
+    r = await client.post(
+        "/api/auth/register",
+        json={
+            "email": "tenant@service.ro",
+            "password": "testpass123",
+            "tenant_name": "Tenant Service",
+            "telefon": "0722000001",
+        },
+    )
+    return r.json()["tenant_id"]