Fix postgres primary/replica races

db/types.go

@@ -11,6 +11,12 @@ type Beginner interface {
 	Begin() (*sql.Tx, error)
 }
 
+// Querier is an interface for types that can execute SQL queries.
+// *sql.DB, *sql.Tx, and *DB all implement this.
+type Querier interface {
+	Query(query string, args ...any) (*sql.Rows, error)
+}
+
 // Host pairs a *sql.DB with the host:port it was opened against.
 type Host struct {
 	Addr    string // "host:port"