refactor: extract _default_command() to eliminate DRY violation

The default wecker.py shell command was duplicated across set_alarm()
and start_ringing(). Extracted a shared _default_command() helper so
both callers use the same definition.

api/schema.py

@@ -28,6 +28,13 @@ def is_wecker_ringing() -> bool:
         return False
 
 
+def _default_command() -> str:
+    """Return the default shell command to run wecker.py."""
+    project_root = Path(__file__).parent.parent.absolute()
+    python_exec = sys.executable
+    return f"cd {project_root} && {python_exec} wecker.py >> wecker.log 2>&1"
+
+
 @strawberry.type
 class Alarm:
     id: str
@@ -68,9 +75,7 @@ class Mutation:
         id: Optional[str] = None,
     ) -> Alarm:
         if command is None:
-            project_root = Path(__file__).parent.parent.absolute()
+            command = _default_command()
-            python_exec = sys.executable
-            command = f"cd {project_root} && {python_exec} wecker.py >> wecker.log 2>&1"
 
         manager = get_manager()
         new_id = manager.set_alarm(
@@ -103,9 +108,7 @@ class Mutation:
         Returns True if started, False if already ringing."""
         if is_wecker_ringing():
             return False
-        project_root = Path(__file__).parent.parent.absolute()
+        cmd = _default_command()
-        python_exec = sys.executable
-        cmd = f"cd {project_root} && {python_exec} wecker.py >> wecker.log 2>&1"
         subprocess.Popen(cmd, shell=True)
         return True