Make count-dependent streams non-sharable, each target gets own instance

Static, gradient, color cycle, and effect streams depend on LED count
and were being reconfigured per-consumer when shared. Now only picture
streams (expensive capture) are shared; count-dependent sources get
per-consumer instances keyed by css_id:target_id.

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

server/src/wled_controller/storage/color_strip_source.py

@@ -33,6 +33,15 @@ class ColorStripSource:
     updated_at: datetime
     description: Optional[str] = None
 
+    @property
+    def sharable(self) -> bool:
+        """Whether multiple consumers can share a single stream instance.
+
+        Count-dependent sources (static, gradient, cycle, effect) return False
+        because each consumer may configure a different LED count.
+        """
+        return False
+
     def to_dict(self) -> dict:
         """Convert source to dictionary. Subclasses extend this."""
         return {
@@ -174,6 +183,11 @@ class PictureColorStripSource(ColorStripSource):
     calibration (LED positions), color correction, smoothing, FPS target.
     """
 
+    @property
+    def sharable(self) -> bool:
+        """Picture streams are expensive (screen capture) and safe to share."""
+        return True
+
     picture_source_id: str = ""
     fps: int = 30
     brightness: float = 1.0          # color correction multiplier (0.0–2.0; 1.0 = unchanged)