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/frontend/timeline/history.js

@@ -0,0 +1,36 @@
+// CommandHistory — undo/redo stack for timeline block mutations
+// Supports min 50 steps. execute() clears redo stack to prevent branching corruption.
+
+export class CommandHistory {
+    constructor(maxSteps = 50) {
+        this.undoStack = [];
+        this.redoStack = [];
+        this.maxSteps = maxSteps;
+    }
+
+    execute(cmd) {
+        cmd.execute();
+        this.undoStack.push(cmd);
+        if (this.undoStack.length > this.maxSteps) this.undoStack.shift();
+        this.redoStack = [];  // clear redo on new action (prevents branching corruption)
+    }
+
+    undo() {
+        const cmd = this.undoStack.pop();
+        if (!cmd) return false;
+        cmd.undo();
+        this.redoStack.push(cmd);
+        return true;
+    }
+
+    redo() {
+        const cmd = this.redoStack.pop();
+        if (!cmd) return false;
+        cmd.execute();
+        this.undoStack.push(cmd);
+        return true;
+    }
+
+    get canUndo() { return this.undoStack.length > 0; }
+    get canRedo() { return this.redoStack.length > 0; }
+}