fix timers

backend/app/api/schemas.py

@@ -29,6 +29,16 @@ class AttachTaskRequest(BaseModel):
 
 class CompleteTimerRequest(BaseModel):
     auto_next: bool = True
+    session_id: int | None = None
+    """Which interval the client believes it is finishing.
+
+    Every client ticks its own local countdown and calls `complete` when it
+    reaches zero, so with two of them open (the phone and the desktop, say)
+    both fire within the same second. Unconditionally, the second call lands
+    on whatever `auto_next` has just started and ends *that* — which is why a
+    break could vanish the instant it began. Sent along, a stale call is a
+    no-op instead. Omitted, the old unconditional behaviour stands.
+    """
 
 
 class TaskCreateRequest(BaseModel):