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/schemas/picture_targets.py

@@ -59,6 +59,7 @@ class PictureTargetCreate(BaseModel):
     keepalive_interval: float = Field(default=1.0, description="Keepalive send interval when screen is static (0.5-5.0s)", ge=0.5, le=5.0)
     state_check_interval: int = Field(default=DEFAULT_STATE_CHECK_INTERVAL, description="Device health check interval (5-600s)", ge=5, le=600)
     min_brightness_threshold: int = Field(default=0, ge=0, le=254, description="Min brightness threshold (0=disabled); below this → off")
+    adaptive_fps: bool = Field(default=False, description="Auto-reduce FPS when device is unresponsive")
     # KC target fields
     picture_source_id: str = Field(default="", description="Picture source ID (for key_colors targets)")
     key_colors_settings: Optional[KeyColorsSettingsSchema] = Field(None, description="Key colors settings (for key_colors targets)")
@@ -78,6 +79,7 @@ class PictureTargetUpdate(BaseModel):
     keepalive_interval: Optional[float] = Field(None, description="Keepalive interval (0.5-5.0s)", ge=0.5, le=5.0)
     state_check_interval: Optional[int] = Field(None, description="Health check interval (5-600s)", ge=5, le=600)
     min_brightness_threshold: Optional[int] = Field(None, ge=0, le=254, description="Min brightness threshold (0=disabled); below this → off")
+    adaptive_fps: Optional[bool] = Field(None, description="Auto-reduce FPS when device is unresponsive")
     # KC target fields
     picture_source_id: Optional[str] = Field(None, description="Picture source ID (for key_colors targets)")
     key_colors_settings: Optional[KeyColorsSettingsSchema] = Field(None, description="Key colors settings (for key_colors targets)")
@@ -99,6 +101,7 @@ class PictureTargetResponse(BaseModel):
     keepalive_interval: float = Field(default=1.0, description="Keepalive interval (s)")
     state_check_interval: int = Field(default=DEFAULT_STATE_CHECK_INTERVAL, description="Health check interval (s)")
     min_brightness_threshold: int = Field(default=0, description="Min brightness threshold (0=disabled)")
+    adaptive_fps: bool = Field(default=False, description="Auto-reduce FPS when device is unresponsive")
     # KC target fields
     picture_source_id: str = Field(default="", description="Picture source ID (key_colors)")
     key_colors_settings: Optional[KeyColorsSettingsSchema] = Field(None, description="Key colors settings")
@@ -152,6 +155,8 @@ class TargetProcessingState(BaseModel):
     device_fps: Optional[int] = Field(None, description="Device-reported FPS (WLED internal refresh rate)")
     device_last_checked: Optional[datetime] = Field(None, description="Last health check time")
     device_error: Optional[str] = Field(None, description="Last health check error")
+    device_streaming_reachable: Optional[bool] = Field(None, description="Device reachable during streaming (HTTP probe)")
+    fps_effective: Optional[int] = Field(None, description="Effective FPS after adaptive reduction")
 
 
 class TargetMetricsResponse(BaseModel):