refactor(status): introduce OrderStatus enum, replace string literals

Centralized order status values in api/app/constants.py via a
str-valued Enum so comparisons keep working. Replaced literals in:
- services: sync_service, sqlite_service, retry_service
- routers: sync, dashboard
- templates: dashboard.html, logs.html
- static JS: shared (ORDER_STATUS mirror), dashboard, logs
- tests: requirements, order_items_overwrite, business_rules

MALFORMED intentionally NOT added — introduced in follow-up PR2
(per-order failure isolation).

Full test suite: 231 unit + 33 e2e pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/routers/dashboard.py

@@ -4,9 +4,11 @@ from fastapi.responses import HTMLResponse
 from pathlib import Path
 
 from ..services import sqlite_service
+from ..constants import OrderStatus
 
 router = APIRouter()
 templates = Jinja2Templates(directory=str(Path(__file__).parent.parent / "templates"))
+templates.env.globals["OrderStatus"] = OrderStatus
 
 @router.get("/", response_class=HTMLResponse)
 async def dashboard(request: Request):