Add admin panel for merchant management

- POST/GET/DELETE /api/admin/users endpoints
- Admin UI at /admin with login, user list, create/delete
- Auth via ADMIN_API_KEY env var (separate from tenant auth)
- Tenants remain scoped to their own store only

api/app/main.py

@@ -7,6 +7,7 @@ from sqlalchemy import text
 
 from .db import Base, engine
 from .config import DONATIONS_ENABLED
+from .admin_routes import router as admin_router
 from .btcpay_routes import router as btcpay_router
 from .routes import router
 
@@ -263,3 +264,4 @@ def startup():
 
 app.include_router(router)
 app.include_router(btcpay_router)
+app.include_router(admin_router)