feat: enhance database queries with error handling and improve SQL statement readability

Co-authored-by: Copilot <copilot@github.com>

backend/app/routers/admin.py

@@ -20,10 +20,18 @@ async def get_stats(_: dict = Depends(require_admin)) -> dict:
     sql_token_count = "SELECT COUNT(*) FROM refresh_tokens"
     sql_tokens_active = "SELECT COUNT(*) FROM refresh_tokens WHERE revoked = false AND expires_at > ?"
     now = datetime.now(timezone.utc)
-    total_users = conn.execute(sql_user_count).fetchone()[0]
+
+    total_users_row = conn.execute(sql_user_count).fetchone()
+    total_users = total_users_row[0] if total_users_row else 0
+
     users_by_role = conn.execute(sql_user_counts).fetchall()
-    total_tokens = conn.execute(sql_token_count).fetchone()[0]
-    active_tokens = conn.execute(sql_tokens_active, [now]).fetchone()[0]
+
+    total_tokens_row = conn.execute(sql_token_count).fetchone()
+    total_tokens = total_tokens_row[0] if total_tokens_row else 0
+
+    active_tokens_row = conn.execute(sql_tokens_active, [now]).fetchone()
+    active_tokens = active_tokens_row[0] if active_tokens_row else 0
+
     return {
         "users": {
             "total": total_users,
@@ -41,7 +49,8 @@ async def get_stats(_: dict = Depends(require_admin)) -> dict:
 async def db_health(_: dict = Depends(require_admin)) -> dict:
     """Verify DuckDB is reachable."""
     conn = get_conn()
-    result = conn.execute("SELECT 1").fetchone()[0]
+    result_row = conn.execute("SELECT 1").fetchone()
+    result = result_row[0] if result_row else 0
     return {"status": "ok" if result == 1 else "error"}
 
 
@@ -54,9 +63,14 @@ async def purge_tokens(_: dict = Depends(require_admin)) -> dict:
     sql_count = "SELECT COUNT(*) FROM refresh_tokens"
     sql_delete = "DELETE FROM refresh_tokens WHERE revoked = true OR expires_at <= ?"
     async with lock:
-        before = conn.execute(sql_count).fetchone()[0]
+        before_row = conn.execute(sql_count).fetchone()
+        before = before_row[0] if before_row else 0
+
         conn.execute(sql_delete, [now])
-        after = conn.execute(sql_count).fetchone()[0]
+
+        after_row = conn.execute(sql_count).fetchone()
+        after = after_row[0] if after_row else 0
+
         return {"deleted": before - after, "remaining": after}