Add adaptive FPS and honest device reachability during streaming

DDP uses fire-and-forget UDP, so when a WiFi device becomes overwhelmed
by sustained traffic, sends appear successful while the device is
actually unreachable. This adds:

- HTTP liveness probe (GET /json/info, 2s timeout) every 10s during
  streaming, exposed as device_streaming_reachable in target state
- Adaptive FPS (opt-in): exponential backoff when device is unreachable,
  gradual recovery when it stabilizes — finds sustainable send rate
- Honest health checks: removed the lie that forced device_online=true
  during streaming; now runs actual health checks regardless
- Target editor toggle, FPS display shows effective rate when throttled,
  health dot reflects streaming reachability, red highlight when
  unreachable
- Auto-backup scheduling support in settings modal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes/system.py

@@ -16,8 +16,12 @@ from pydantic import BaseModel
 
 from wled_controller import __version__
 from wled_controller.api.auth import AuthRequired
-from wled_controller.api.dependencies import get_processor_manager
+from wled_controller.api.dependencies import get_auto_backup_engine, get_processor_manager
 from wled_controller.api.schemas.system import (
+    AutoBackupSettings,
+    AutoBackupStatusResponse,
+    BackupFileInfo,
+    BackupListResponse,
     DisplayInfo,
     DisplayListResponse,
     GpuInfo,
@@ -27,6 +31,7 @@ from wled_controller.api.schemas.system import (
     RestoreResponse,
     VersionResponse,
 )
+from wled_controller.core.backup.auto_backup import AutoBackupEngine
 from wled_controller.config import get_config
 from wled_controller.core.capture.screen_capture import get_available_displays
 from wled_controller.utils import atomic_write_json, get_logger
@@ -348,6 +353,93 @@ async def restore_config(
     )
 
 
+# ---------------------------------------------------------------------------
+# Auto-backup settings & saved backups
+# ---------------------------------------------------------------------------
+
+
+@router.get(
+    "/api/v1/system/auto-backup/settings",
+    response_model=AutoBackupStatusResponse,
+    tags=["System"],
+)
+async def get_auto_backup_settings(
+    _: AuthRequired,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """Get auto-backup settings and status."""
+    return engine.get_settings()
+
+
+@router.put(
+    "/api/v1/system/auto-backup/settings",
+    response_model=AutoBackupStatusResponse,
+    tags=["System"],
+)
+async def update_auto_backup_settings(
+    _: AuthRequired,
+    body: AutoBackupSettings,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """Update auto-backup settings (enable/disable, interval, max backups)."""
+    return await engine.update_settings(
+        enabled=body.enabled,
+        interval_hours=body.interval_hours,
+        max_backups=body.max_backups,
+    )
+
+
+@router.get(
+    "/api/v1/system/backups",
+    response_model=BackupListResponse,
+    tags=["System"],
+)
+async def list_backups(
+    _: AuthRequired,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """List all saved backup files."""
+    backups = engine.list_backups()
+    return BackupListResponse(
+        backups=[BackupFileInfo(**b) for b in backups],
+        count=len(backups),
+    )
+
+
+@router.get("/api/v1/system/backups/{filename}", tags=["System"])
+def download_saved_backup(
+    filename: str,
+    _: AuthRequired,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """Download a specific saved backup file."""
+    try:
+        path = engine.get_backup_path(filename)
+    except (ValueError, FileNotFoundError) as e:
+        raise HTTPException(status_code=404, detail=str(e))
+
+    content = path.read_bytes()
+    return StreamingResponse(
+        io.BytesIO(content),
+        media_type="application/json",
+        headers={"Content-Disposition": f'attachment; filename="{filename}"'},
+    )
+
+
+@router.delete("/api/v1/system/backups/{filename}", tags=["System"])
+async def delete_saved_backup(
+    filename: str,
+    _: AuthRequired,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """Delete a specific saved backup file."""
+    try:
+        engine.delete_backup(filename)
+    except (ValueError, FileNotFoundError) as e:
+        raise HTTPException(status_code=404, detail=str(e))
+    return {"status": "deleted", "filename": filename}
+
+
 # ---------------------------------------------------------------------------
 # ADB helpers (for Android / scrcpy engine)
 # ---------------------------------------------------------------------------