chore: track pubspec.lock and pin sqlite3 to ^3.1.5

pubspec.lock was incorrectly gitignored — this is a Flutter app, not a
package, so the lockfile should be committed for reproducible builds.
Without it, CI resolved drift to its minimum (2.20.3) which constrains
sqlite3 to 2.x, causing dart analyze to disagree on whether
Database.close() exists vs the local environment using 3.3.1.

Also pins sqlite3: ^3.1.5 explicitly in pubspec.yaml as belt-and-
suspenders so the constraint is visible without reading the lockfile.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,7 +3,6 @@ coverage/
 .dart_tool/
 .dart-tool/
 .packages
-pubspec.lock
 build/
 *.g.dart
 *.freezed.dart

pubspec.yaml

@@ -74,7 +74,7 @@ dev_dependencies:
   mockito: ^5.4.4
   fake_async: ^1.3.1
   path_provider_platform_interface: ^2.1.2
-  sqlite3: any  # used directly in test/unit/db_test_helper.dart
+  sqlite3: ^3.1.5  # used directly in test/unit/db_test_helper.dart; 3.x required for Database.close()
   url_launcher_platform_interface: ^2.3.2
   plugin_platform_interface: ^2.1.8