Phase 6: remote backup destinations — SFTP & S3 (0.6.0)

- BackupDestination model + backup_destination_service (SFTP via paramiko,
  S3-compatible via boto3): upload/list/download/delete/test.
- routers/destinations.py: destinations CRUD (secrets masked, merge-on-update),
  test, list/delete remote backups. backups.py: POST /{id}/backup/push and
  POST /restore-from (download from a destination + restore, volumes included).
- Frontend: Settings → Backup destinations (SFTP/S3 forms + test); Backup dialog
  can push to a destination; Restore dialog can pick a destination + backup.
- deps: paramiko 3.5.0, boto3 1.35.99.

Verified end-to-end against live MinIO + atmoz/sftp: create/test destinations,
push (incl. volumes), list, restore-from to a fresh stack (volume data intact),
delete remote backup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/main.py

@@ -18,6 +18,7 @@ from routers import (
     audit,
     auth,
     backups,
+    destinations,
     editor,
     images,
     ports,
@@ -49,7 +50,7 @@ async def lifespan(app: FastAPI):
     update_task.cancel()
 
 
-app = FastAPI(title="StackPilot", version="0.5.0", lifespan=lifespan)
+app = FastAPI(title="StackPilot", version="0.6.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
@@ -79,6 +80,7 @@ app.include_router(templates.router)
 app.include_router(audit.router)
 app.include_router(settings_router.router)
 app.include_router(backups.router)
+app.include_router(destinations.router)
 app.include_router(agents.router)
 app.include_router(ws.router)