fix: dedupe existing rules + unique index to prevent duplicates

- Add unique index on alert_rules.name in setup_indexes()
- seed_default_rules() now removes duplicates by name before upserting
- Keeps the oldest document (_id ascending) when deduping

backend/database.py

@@ -23,6 +23,7 @@ def setup_indexes(max_retries: int = 5, delay: float = 2.0):
             events_collection.create_index([("service", ASCENDING), ("timestamp", DESCENDING)])
             events_collection.create_index("id")
             saved_searches_collection.create_index([("created_by", ASCENDING), ("created_at", DESCENDING)])
+            db["alert_rules"].create_index("name", unique=True)
             events_collection.create_index(
                 [("actor_display", TEXT), ("raw_text", TEXT), ("operation", TEXT)],
                 name="text_search_index",