Check unique constraint errors when manually inserting migrations (#2712)

This should avoid unnecessary logging on startup if the migration (were
we need `InsertMigration`) was already executed.
This now checks for "unique constraint errors" for SQLite and Postgres
and fails the startup process if the migration couldn't be manually
inserted for some other reason.

internal/sqlutil/migrate.go

@@ -155,5 +155,10 @@ func InsertMigration(ctx context.Context, db *sql.DB, migrationName string) erro
 		time.Now().Format(time.RFC3339),
 		internal.VersionString(),
 	)
+	// If the migration was already executed, we'll get a unique constraint error,
+	// return nil instead, to avoid unnecessary logging.
+	if IsUniqueConstraintViolationErr(err) {
+		return nil
+	}
 	return err
 }