M5: end-to-end test matrix and CI

- tests/integration/roundtrip.sh: key, kit code and confirmation,
  location, full and unchanged incremental backup, restore new-folder and
  replace (files identical, extra file in the trash), database dump and
  restore, retention, trash and restore from trash, no files_external
  errors in the log
- tests/integration/matrix.sh: fresh Nextcloud per version and database
  (SQLite, MariaDB, PostgreSQL), removes only its own containers and
  their volumes; locally green on NC32 and NC35 with all three databases
- GitHub Actions: unit tests on PHP 8.2-8.4, round trip on NC32-35 x
  SQLite/MariaDB/PostgreSQL

Fixes found by the matrix:
- files_external is no longer booted when it is disabled: only its
  classes are loaded and a private BackendService gets its backends from
  its Application class (booting registered its mount provider, which
  failed on installations where its tables were never created)
- SQLite: the database file is excluded from the file backup (it is
  covered by the logical dump and copying the live file is inconsistent)
- occ backup:key:confirm accepts --code and --confirm for scripts

spikes/03-db-dump.php

@@ -48,9 +48,11 @@ $seqStates = static function () use ($db, $prefix): array {
 	return $out;
 };
 // Let one sequence run ahead of the data (deleted rows, rollbacks): MAX+1 would be wrong.
-$fcSeq = $db->fetchOne("SELECT pg_get_serial_sequence(?, 'fileid')", [$prefix . 'filecache']);
-for ($i = 0; $i < 50; $i++) {
-	$db->fetchOne('SELECT nextval(CAST(? AS regclass))', [$fcSeq]);
+if ($db->getDatabasePlatform() instanceof \Doctrine\DBAL\Platforms\PostgreSQLPlatform) {
+	$fcSeq = $db->fetchOne("SELECT pg_get_serial_sequence(?, 'fileid')", [$prefix . 'filecache']);
+	for ($i = 0; $i < 50; $i++) {
+		$db->fetchOne('SELECT nextval(CAST(? AS regclass))', [$fcSeq]);
+	}
 }
 $seqBefore = $seqStates();
 $before = [];
@@ -122,7 +124,8 @@ if ($mismatch) {
 // Sequences must be restored exactly (PostgreSQL), including one that ran ahead of the data.
 $seqAfter = $seqStates();
 $diff = array_filter(array_keys($seqBefore), fn ($k) => ($seqAfter[$k] ?? null) !== $seqBefore[$k]);
-$check(sprintf('all %d sequences restored exactly (filecache seq 50 ahead of data)', count($seqBefore)), $diff === [] && count($seqBefore) > 0);
+$isPg = $db->getDatabasePlatform() instanceof \Doctrine\DBAL\Platforms\PostgreSQLPlatform;
+$check(sprintf('all %d sequences restored exactly%s', count($seqBefore), $isPg ? ' (filecache seq 50 ahead of data)' : ' (n/a on this database)'), $diff === [] && (!$isPg || count($seqBefore) > 0));
 if ($diff) {
 	foreach (array_slice($diff, 0, 5) as $k) {
 		echo "$k before=" . json_encode($seqBefore[$k]) . ' after=' . json_encode($seqAfter[$k] ?? null) . "\n";