milestone 26: upstream health answers for the selected period

src/cli.zig

@@ -341,8 +341,10 @@ pub const DataDir = struct {
     }
 
     /// An additional connection to a `querylog.db` that `openQuerylogDb` has
-    /// already established. A running server needs two — the log writer and the
-    /// retention pass each own one (`retention.zig`'s contract).
+    /// already established. A running server needs three background ones — the
+    /// log writer, the retention pass and the upstream-history flush each own
+    /// one (`retention.zig`'s contract) — plus a fourth for the web task when
+    /// the web interface is enabled.
     pub fn reopenQuerylogDb(self: *const DataDir, io: std.Io) !db.Db {
         _ = io;
         var database = try db.Db.open(self.querylog_db_path, .{ .mode = .read_write_existing });