Fix API calls by all workers

Quite a complex commit by GPT. I'll rewrite it somewhen

blueprints/risdeveau/modules/api/scheduler_guard.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import os
+import fcntl
+from typing import Optional
+
+_LOCK_FD: Optional[int] = None  # keep fd open for process lifetime
+
+
+def should_start_scheduler() -> bool:
+    """Return True in exactly one process (the scheduler owner).
+
+    Under gunicorn --workers N, code is imported in N processes. If APScheduler
+    starts at import time, you get N schedulers => N× API calls.
+
+    We prevent that by taking a non-blocking exclusive lock on a lockfile.
+    """
+    global _LOCK_FD
+    if _LOCK_FD is not None:
+        return True
+
+    lock_path = os.environ.get("LAIR_SCHED_LOCK", "/tmp/lair-scheduler.lock")
+    fd = os.open(lock_path, os.O_CREAT | os.O_RDWR, 0o644)
+    try:
+        fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        _LOCK_FD = fd
+        return True
+    except BlockingIOError:
+        os.close(fd)
+        return False
+    except Exception:
+        try:
+            os.close(fd)
+        finally:
+            return False