Add value source test modal, auto-gain, brightness always-show, shared value streams

- Add real-time value source test: WebSocket endpoint streams get_value() at
  ~20Hz, frontend renders scrolling time-series chart with min/max/current stats
- Add auto-gain for audio value sources: rolling peak normalization with slow
  decay, sensitivity range increased to 0.1-20.0
- Always show brightness overlay on LED preview when brightness source is set
- Refactor ValueStreamManager to shared ref-counted streams (value streams
  produce scalars, not LED-count-dependent, so sharing is correct)
- Simplify acquire/release API: remove consumer_id parameter since streams
  are no longer consumer-dependent

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

server/src/wled_controller/storage/value_source.py

@@ -45,6 +45,7 @@ class ValueSource:
             "mode": None,
             "sensitivity": None,
             "smoothing": None,
+            "auto_gain": None,
             "schedule": None,
             "picture_source_id": None,
             "scene_behavior": None,
@@ -93,6 +94,7 @@ class ValueSource:
                 smoothing=float(data.get("smoothing") or 0.3),
                 min_value=float(data.get("min_value") or 0.0),
                 max_value=float(data["max_value"]) if data.get("max_value") is not None else 1.0,
+                auto_gain=bool(data.get("auto_gain", False)),
             )
 
         if source_type == "adaptive_time":
@@ -171,10 +173,11 @@ class AudioValueSource(ValueSource):
 
     audio_source_id: str = ""    # references an audio source (mono or multichannel)
     mode: str = "rms"            # rms | peak | beat
-    sensitivity: float = 1.0     # gain multiplier (0.1–5.0)
+    sensitivity: float = 1.0     # gain multiplier (0.1–20.0)
     smoothing: float = 0.3       # temporal smoothing (0.0–1.0)
     min_value: float = 0.0       # minimum output (0.0–1.0)
     max_value: float = 1.0       # maximum output (0.0–1.0)
+    auto_gain: bool = False      # auto-normalize audio levels to full range
 
     def to_dict(self) -> dict:
         d = super().to_dict()
@@ -184,6 +187,7 @@ class AudioValueSource(ValueSource):
         d["smoothing"] = self.smoothing
         d["min_value"] = self.min_value
         d["max_value"] = self.max_value
+        d["auto_gain"] = self.auto_gain
         return d