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/sync/schemas.py

@@ -0,0 +1,18 @@
+from pydantic import BaseModel
+
+
+class SyncOperation(BaseModel):
+    table: str
+    id: str
+    operation: str  # INSERT | UPDATE | DELETE
+    data: dict = {}
+    timestamp: str
+
+
+class SyncPushRequest(BaseModel):
+    operations: list[SyncOperation]
+
+
+class SyncPushResponse(BaseModel):
+    applied: int
+    conflicts: list = []