fix(db): prevent concurrent startup locks

test/sqlite_helpers_test.dart

@@ -0,0 +1,91 @@
+import 'dart:async';
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/services/sqlite_helpers.dart';
+
+void main() {
+  group('SingleFlightInitializer', () {
+    test('coalesces concurrent initialization and caches the result', () async {
+      final initializer = SingleFlightInitializer<int>();
+      final gate = Completer<void>();
+      var calls = 0;
+
+      Future<int> create() async {
+        calls++;
+        await gate.future;
+        return 42;
+      }
+
+      final first = initializer.getOrCreate(create);
+      final second = initializer.getOrCreate(create);
+
+      expect(identical(first, second), isTrue);
+      expect(calls, 1);
+
+      gate.complete();
+      expect(await Future.wait([first, second]), [42, 42]);
+      expect(await initializer.getOrCreate(create), 42);
+      expect(calls, 1);
+    });
+
+    test('allows retry after initialization fails', () async {
+      final initializer = SingleFlightInitializer<int>();
+      var calls = 0;
+
+      Future<int> create() {
+        calls++;
+        if (calls == 1) throw StateError('first attempt failed');
+        return Future<int>.value(7);
+      }
+
+      await expectLater(initializer.getOrCreate(create), throwsStateError);
+      expect(await initializer.getOrCreate(create), 7);
+      expect(calls, 2);
+    });
+
+    test('reset permits a fresh initialization', () async {
+      final initializer = SingleFlightInitializer<int>();
+      var value = 1;
+
+      expect(await initializer.getOrCreate(() async => value), 1);
+      value = 2;
+      expect(await initializer.getOrCreate(() async => value), 1);
+
+      initializer.reset();
+      expect(await initializer.getOrCreate(() async => value), 2);
+    });
+  });
+
+  group('SQLite startup configuration', () {
+    final source = File('lib/services/sqlite_helpers.dart').readAsStringSync();
+
+    test('waits for transient locks before configuring the connection', () {
+      final configureIndex = source.indexOf('onConfigure:');
+      final busyTimeoutIndex = source.indexOf('PRAGMA busy_timeout');
+      final journalModeIndex = source.indexOf('PRAGMA journal_mode');
+
+      expect(configureIndex, greaterThanOrEqualTo(0));
+      expect(busyTimeoutIndex, greaterThan(configureIndex));
+      expect(busyTimeoutIndex, lessThan(journalModeIndex));
+    });
+
+    test('sets incremental auto-vacuum only before app tables exist', () {
+      final schemaCheckIndex = source.indexOf('FROM sqlite_master');
+      final emptySchemaGuardIndex = source.indexOf('if (tables.isEmpty)');
+      final autoVacuumMatches = RegExp(
+        r"PRAGMA auto_vacuum = INCREMENTAL",
+      ).allMatches(source).toList();
+      final onCreateIndex = source.indexOf('onCreate:');
+
+      expect(autoVacuumMatches, hasLength(1));
+      expect(schemaCheckIndex, greaterThanOrEqualTo(0));
+      expect(emptySchemaGuardIndex, greaterThan(schemaCheckIndex));
+      expect(
+        autoVacuumMatches.single.start,
+        greaterThan(emptySchemaGuardIndex),
+      );
+      expect(autoVacuumMatches.single.start, lessThan(onCreateIndex));
+    });
+  });
+}