fix: dedupe host_silence problems under concurrent scans (0.4.11)

Treat acknowledged as active, dedupe by hostname, add PostgreSQL advisory lock and a unique index. Migration closes existing duplicate open problems.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/problems.py

@@ -51,7 +51,7 @@ def open_or_append_problem(
         if ref.tzinfo is None:
             ref = ref.replace(tzinfo=timezone.utc)
         problem, created = open_or_refresh_host_silence_problem(
-            db, event.host_id, match, now=ref
+            db, event.host_id, match, now=ref, hostname=event.host.hostname if event.host else None
         )
         if problem is None:
             return None, False