Add CRM order queue: live sync, view modal, receipt tabs, delete

Wires the CRM (exoCRM GetOrders) into the dashboard as a locally
persisted order queue instead of the previous static mockup:

- CrmClient Protocol + ExoCrmClient/StubCrmClient for the CRM's
  signed JSON-RPC API
- Order model + migration, synced from CRM on each queue view;
  soft-deleted orders stay hidden across re-syncs
- GET/DELETE /api/v1/orders with "no receipt"/"receipt issued" tabs
  (the latter is empty until Checkbox fiscalization lands)
- Dashboard: real order list, item-detail modal, tab switcher,
  one-click delete

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/db/models/__init__.py

@@ -6,12 +6,14 @@ Alembic автогенерирует миграции по `Base.metadata`, по
 
 from app.db.base import Base
 from app.db.models.audit import AuditAction, AuditLog
+from app.db.models.order import Order
 from app.db.models.user import RefreshToken, User, UserRole
 
 __all__ = [
     "AuditAction",
     "AuditLog",
     "Base",
+    "Order",
     "RefreshToken",
     "User",
     "UserRole",