Add support for pg_stat_statements configuration in Targets

This commit introduces a `use_pg_stat_statements` flag for targets, allowing users to enable or disable the use of `pg_stat_statements` for query insights. It includes database schema changes, backend logic, and UI updates to manage this setting in both creation and editing workflows.

backend/app/services/collector.py

@@ -106,18 +106,19 @@ async def collect_target(target: Target) -> None:
             cache_hit_ratio = stat_db["blks_hit"] / (stat_db["blks_hit"] + stat_db["blks_read"])
 
         query_rows = []
-        try:
-            query_rows = await conn.fetch(
-                """
-                SELECT queryid::text, calls, total_exec_time, mean_exec_time, rows, left(query, 2000) AS query_text
-                FROM pg_stat_statements
-                ORDER BY total_exec_time DESC
-                LIMIT 20
-                """
-            )
-        except Exception:
-            # Extension may be disabled on monitored instance.
-            query_rows = []
+        if target.use_pg_stat_statements:
+            try:
+                query_rows = await conn.fetch(
+                    """
+                    SELECT queryid::text, calls, total_exec_time, mean_exec_time, rows, left(query, 2000) AS query_text
+                    FROM pg_stat_statements
+                    ORDER BY total_exec_time DESC
+                    LIMIT 20
+                    """
+                )
+            except Exception:
+                # Extension may be disabled on monitored instance.
+                query_rows = []
 
         async with SessionLocal() as db:
             await _store_metric(db, target.id, "connections_total", activity["total_connections"], {})