feat: Key batch management (Key Manager service, API, frontend) (Phase 6)

Backend service for reading/writing profile .env files with masking,
auto-backup, batch overwrite, batch scan, diff view, and CSV export.
Owner/operator-only API endpoints. Frontend KeysPage with batch-overwrite
modal, batch-scan results, diff view with highlight, and CSV download.
Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/main.py

@@ -4,7 +4,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.database import engine, Base
-from app.api import auth, users, audit, dashboard, gateway, health, cron
+from app.api import auth, users, audit, dashboard, gateway, health, cron, keys
 from app.middleware.audit import AuditMiddleware
 
 
@@ -38,6 +38,7 @@ app.include_router(dashboard.router, prefix="/api/v1")
 app.include_router(gateway.router, prefix="/api/v1")
 app.include_router(health.router, prefix="/api/v1")
 app.include_router(cron.router, prefix="/api/v1")
+app.include_router(keys.router, prefix="/api/v1")
 
 
 @app.get("/api/v1/health")