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/storage/picture_target_store.py

@@ -100,6 +100,7 @@ class PictureTargetStore:
         keepalive_interval: float = 1.0,
         state_check_interval: int = DEFAULT_STATE_CHECK_INTERVAL,
         min_brightness_threshold: int = 0,
+        adaptive_fps: bool = False,
         key_colors_settings: Optional[KeyColorsSettings] = None,
         description: Optional[str] = None,
         picture_source_id: str = "",
@@ -133,6 +134,7 @@ class PictureTargetStore:
                 keepalive_interval=keepalive_interval,
                 state_check_interval=state_check_interval,
                 min_brightness_threshold=min_brightness_threshold,
+                adaptive_fps=adaptive_fps,
                 description=description,
                 auto_start=auto_start,
                 created_at=now,
@@ -170,6 +172,7 @@ class PictureTargetStore:
         keepalive_interval: Optional[float] = None,
         state_check_interval: Optional[int] = None,
         min_brightness_threshold: Optional[int] = None,
+        adaptive_fps: Optional[bool] = None,
         key_colors_settings: Optional[KeyColorsSettings] = None,
         description: Optional[str] = None,
         auto_start: Optional[bool] = None,
@@ -199,6 +202,7 @@ class PictureTargetStore:
             keepalive_interval=keepalive_interval,
             state_check_interval=state_check_interval,
             min_brightness_threshold=min_brightness_threshold,
+            adaptive_fps=adaptive_fps,
             key_colors_settings=key_colors_settings,
             description=description,
             auto_start=auto_start,

server/src/wled_controller/storage/wled_picture_target.py

@@ -20,6 +20,7 @@ class WledPictureTarget(PictureTarget):
     keepalive_interval: float = 1.0 # seconds between keepalive sends when screen is static
     state_check_interval: int = DEFAULT_STATE_CHECK_INTERVAL
     min_brightness_threshold: int = 0  # brightness below this → 0 (disabled when 0)
+    adaptive_fps: bool = False          # auto-reduce FPS when device is unresponsive
 
     def register_with_manager(self, manager) -> None:
         """Register this WLED target with the processor manager."""
@@ -33,6 +34,7 @@ class WledPictureTarget(PictureTarget):
                 state_check_interval=self.state_check_interval,
                 brightness_value_source_id=self.brightness_value_source_id,
                 min_brightness_threshold=self.min_brightness_threshold,
+                adaptive_fps=self.adaptive_fps,
             )
 
     def sync_with_manager(self, manager, *, settings_changed: bool,
@@ -46,6 +48,7 @@ class WledPictureTarget(PictureTarget):
                 "keepalive_interval": self.keepalive_interval,
                 "state_check_interval": self.state_check_interval,
                 "min_brightness_threshold": self.min_brightness_threshold,
+                "adaptive_fps": self.adaptive_fps,
             })
         if css_changed:
             manager.update_target_css(self.id, self.color_strip_source_id)
@@ -57,7 +60,7 @@ class WledPictureTarget(PictureTarget):
     def update_fields(self, *, name=None, device_id=None, color_strip_source_id=None,
                       brightness_value_source_id=None,
                       fps=None, keepalive_interval=None, state_check_interval=None,
-                      min_brightness_threshold=None,
+                      min_brightness_threshold=None, adaptive_fps=None,
                       description=None, auto_start=None, **_kwargs) -> None:
         """Apply mutable field updates for WLED targets."""
         super().update_fields(name=name, description=description, auto_start=auto_start)
@@ -75,6 +78,8 @@ class WledPictureTarget(PictureTarget):
             self.state_check_interval = state_check_interval
         if min_brightness_threshold is not None:
             self.min_brightness_threshold = min_brightness_threshold
+        if adaptive_fps is not None:
+            self.adaptive_fps = adaptive_fps
 
     @property
     def has_picture_source(self) -> bool:
@@ -90,6 +95,7 @@ class WledPictureTarget(PictureTarget):
         d["keepalive_interval"] = self.keepalive_interval
         d["state_check_interval"] = self.state_check_interval
         d["min_brightness_threshold"] = self.min_brightness_threshold
+        d["adaptive_fps"] = self.adaptive_fps
         return d
 
     @classmethod
@@ -116,6 +122,7 @@ class WledPictureTarget(PictureTarget):
             keepalive_interval=data.get("keepalive_interval", data.get("standby_interval", 1.0)),
             state_check_interval=data.get("state_check_interval", DEFAULT_STATE_CHECK_INTERVAL),
             min_brightness_threshold=data.get("min_brightness_threshold", 0),
+            adaptive_fps=data.get("adaptive_fps", False),
             description=data.get("description"),
             auto_start=data.get("auto_start", False),
             created_at=datetime.fromisoformat(data.get("created_at", datetime.utcnow().isoformat())),