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/static/js/features/targets.js

@@ -868,7 +868,7 @@ export function createTargetCard(target, deviceMap, colorStripSourceMap, valueSo
             </div>
             <div id="led-preview-panel-${target.id}" class="led-preview-panel" style="display:${ledPreviewWebSockets[target.id] ? '' : 'none'}">
                 <canvas id="led-preview-canvas-${target.id}" class="led-preview-canvas"></canvas>
-                <span id="led-preview-brightness-${target.id}" class="led-preview-brightness" style="display:none"></span>
+                <span id="led-preview-brightness-${target.id}" class="led-preview-brightness" style="display:none"${bvsId ? ' data-has-bvs="1"' : ''}></span>
             </div>
             <div class="card-actions">
                 ${isProcessing ? `
@@ -1071,11 +1071,11 @@ function connectLedPreviewWS(targetId) {
                 _ledPreviewLastFrame[targetId] = frame;
                 const canvas = document.getElementById(`led-preview-canvas-${targetId}`);
                 if (canvas) _renderLedStrip(canvas, frame);
-                // Show brightness label when below 100%
+                // Show brightness label: always when a brightness source is set, otherwise only below 100%
                 const bLabel = document.getElementById(`led-preview-brightness-${targetId}`);
                 if (bLabel) {
                     const pct = Math.round(brightness / 255 * 100);
-                    if (pct < 100) {
+                    if (pct < 100 || bLabel.dataset.hasBvs) {
                         bLabel.textContent = `☀ ${pct}%`;
                         bLabel.style.display = '';
                     } else {