feat(04-02): command pattern + CommandHistory + block CRUD backend

- CommandHistory class with execute/undo/redo, 50-step max, redo-clear on new action
- PlaceBlockCommand, MoveBlockCommand, ResizeBlockCommand, DeleteBlockCommand
- setCurrentShowId() module-level function for backend sync routing
- syncBlock() fire-and-forget fetch helper for backend sync
- POST/PATCH/DELETE /api/shows/{show_id}/tracks/{device_id}/blocks endpoints
- timeline router registered in main.py with prefix=/api

lightsync/api/timeline.py

@@ -0,0 +1,86 @@
+from uuid import UUID
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+from typing import Any
+import lightsync.main as state
+
+router = APIRouter(tags=["timeline"])
+
+
+class BlockCreate(BaseModel):
+    id: str  # UUID as string from frontend crypto.randomUUID()
+    timestamp: float
+    duration: float = 4.0
+    animation: str | None = None
+    params: dict[str, Any] = {}
+
+
+class BlockUpdate(BaseModel):
+    timestamp: float | None = None
+    duration: float | None = None
+    animation: str | None = None
+    params: dict[str, Any] | None = None
+
+
+@router.post("/shows/{show_id}/tracks/{device_id}/blocks", status_code=201)
+async def create_block(show_id: str, device_id: str, block: BlockCreate):
+    show = await state.show_store.load(show_id)
+    if show is None:
+        raise HTTPException(404, "Show not found")
+    track = next((t for t in show.tracks if str(t.device_id) == device_id), None)
+    if track is None:
+        # Auto-create track for this device
+        from lightsync.models.show import TrackModel, CueModel
+        cue = CueModel(
+            id=UUID(block.id), timestamp=block.timestamp,
+            duration=block.duration, animation=block.animation,
+            params=block.params
+        )
+        track = TrackModel(device_id=UUID(device_id), cues=[cue])
+        show.tracks.append(track)
+    else:
+        from lightsync.models.show import CueModel
+        cue = CueModel(
+            id=UUID(block.id), timestamp=block.timestamp,
+            duration=block.duration, animation=block.animation,
+            params=block.params
+        )
+        track.cues.append(cue)
+    await state.show_store.save(show)
+    return {"status": "ok"}
+
+
+@router.patch("/shows/{show_id}/tracks/{device_id}/blocks/{block_id}")
+async def update_block(show_id: str, device_id: str, block_id: str, update: BlockUpdate):
+    show = await state.show_store.load(show_id)
+    if show is None:
+        raise HTTPException(404, "Show not found")
+    track = next((t for t in show.tracks if str(t.device_id) == device_id), None)
+    if track is None:
+        raise HTTPException(404, "Track not found")
+    cue = next((c for c in track.cues if str(c.id) == block_id), None)
+    if cue is None:
+        raise HTTPException(404, "Block not found")
+    if update.timestamp is not None:
+        cue.timestamp = update.timestamp
+    if update.duration is not None:
+        cue.duration = update.duration
+    if update.animation is not None:
+        cue.animation = update.animation
+    if update.params is not None:
+        cue.params = update.params
+    await state.show_store.save(show)
+    return {"status": "ok"}
+
+
+@router.delete("/shows/{show_id}/tracks/{device_id}/blocks/{block_id}")
+async def delete_block(show_id: str, device_id: str, block_id: str):
+    show = await state.show_store.load(show_id)
+    if show is None:
+        raise HTTPException(404, "Show not found")
+    track = next((t for t in show.tracks if str(t.device_id) == device_id), None)
+    if track is None:
+        raise HTTPException(404, "Track not found")
+    track.cues = [c for c in track.cues if str(c.id) != block_id]
+    await state.show_store.save(show)
+    return {"status": "ok"}