relay: fix UnboundLocalError in dedupe pruning (rebind made the dict local)

Every webhook died on 'cannot access local variable _last_sent' because
the prune comprehension rebound the module global inside the function.
Prune in place instead.

scripts/signal-relay.py

@@ -106,7 +106,12 @@ def handle_alertmanager(payload):
         return
     now = time.monotonic()
     key = dedupe_key(alerts)
-    _last_sent = {k: v for k, v in _last_sent.items() if now - v < DEDUPE_SECONDS}
+    # Prune in place: rebinding _last_sent here would make it local to this
+    # function and the read above the assignment raises UnboundLocalError
+    # (measured 2026-09-17: every webhook died on it).
+    stale = [k for k, v in _last_sent.items() if now - v >= DEDUPE_SECONDS]
+    for k in stale:
+        del _last_sent[k]
     if key in _last_sent:
         log(f"suppressed duplicate: {key[0]}")
         return