autoresearch iter 22: delete lint suppressions that suppress nothing

24 of the 96 nolint directives were dead: they covered findings that no
longer exist. A stale suppression is not inert — it silently claims any
future finding for that linter in that scope, so a real problem raised
there would vanish without anyone noticing. Explanatory prose was kept as
ordinary comments.

Two directives proved load-bearing under the project gate even though
nolintlint reported them unused, and removing them exposed verified
contextcheck false positives: App.Run does forward a sigCtx derived from
the caller's context to Start, and the migration lock renewal must keep
its own deadline because the task context may already be canceled. Both
were restored, narrowed to the live linter, and given the reason the
originals lacked.

backend/plugins/domain/admin/repository/db.go

@@ -327,7 +327,7 @@ func GetPostgresInfo(ctx context.Context) model.DatabaseInfoResponse {
 
 // OpenSQLiteExportFile opens the active SQLite database file together with its stat info.
 func OpenSQLiteExportFile() (*os.File, os.FileInfo, error) {
-	//nolint:gosec // export db file path is trusted
+	// export db file path is trusted
 	f, err := os.Open(sqliteDatabasePath())
 	if err != nil {
 		return nil, nil, fmt.Errorf("%s: %w", errs.ErrOpenDatabaseFileFailed, err)