Add sync clock entity for synchronized animation timing

Introduces Synchronization Clocks — shared, controllable time bases
that CSS sources can optionally reference for synchronized animation.

Backend:
- New SyncClock dataclass, JSON store, Pydantic schemas, REST API
- Runtime clock with thread-safe pause/resume/reset and speed control
- Ref-counted runtime pool with eager creation for API control
- clock_id field on all ColorStripSource types
- Stream integration: clock time/speed replaces source-local values
- Paused clock skips rendering (saves CPU + stops frame pushes)
- Included in backup/restore via STORE_MAP

Frontend:
- Sync Clocks tab in Streams section with cards and controls
- Clock dropdown in CSS editor (hidden speed slider when clock set)
- Clock crosslink badge on CSS source cards (replaces speed badge)
- Targets tab uses DataCache for picture/audio sources and sync clocks

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

server/src/wled_controller/api/schemas/sync_clocks.py

@@ -0,0 +1,42 @@
+"""Sync clock schemas (CRUD + control)."""
+
+from datetime import datetime
+from typing import List, Optional
+
+from pydantic import BaseModel, Field
+
+
+class SyncClockCreate(BaseModel):
+    """Request to create a synchronization clock."""
+
+    name: str = Field(description="Clock name", min_length=1, max_length=100)
+    speed: float = Field(default=1.0, description="Speed multiplier (0.1–10.0)", ge=0.1, le=10.0)
+    description: Optional[str] = Field(None, description="Optional description", max_length=500)
+
+
+class SyncClockUpdate(BaseModel):
+    """Request to update a synchronization clock."""
+
+    name: Optional[str] = Field(None, description="Clock name", min_length=1, max_length=100)
+    speed: Optional[float] = Field(None, description="Speed multiplier (0.1–10.0)", ge=0.1, le=10.0)
+    description: Optional[str] = Field(None, description="Optional description", max_length=500)
+
+
+class SyncClockResponse(BaseModel):
+    """Synchronization clock response."""
+
+    id: str = Field(description="Clock ID")
+    name: str = Field(description="Clock name")
+    speed: float = Field(description="Speed multiplier")
+    description: Optional[str] = Field(None, description="Description")
+    is_running: bool = Field(True, description="Whether clock is currently running")
+    elapsed_time: float = Field(0.0, description="Current elapsed time in seconds")
+    created_at: datetime = Field(description="Creation timestamp")
+    updated_at: datetime = Field(description="Last update timestamp")
+
+
+class SyncClockListResponse(BaseModel):
+    """List of synchronization clocks."""
+
+    clocks: List[SyncClockResponse] = Field(description="List of sync clocks")
+    count: int = Field(description="Number of clocks")