Two categories of fixes:
1. Use defer s.Close() instead of explicit s.Close() to prevent session
leaks when require.FailNow() triggers runtime.Goexit(), which skips
explicit close calls but runs deferred functions. Leaked sessions
hold SQLite write locks that block all subsequent fixture loading.
2. Add s.Commit() before db.AssertExists/db.AssertMissing calls. These
assertion helpers query via the global engine (not the test session),
so they cannot see uncommitted data from the session's transaction.
For block-scoped sessions (kanban_task_bucket_test.go), wrap each block
in an anonymous function so defer runs at block boundary rather than
deferring to the enclosing test function.
Verify that deleting a parent project atomically deletes all child
projects, including archived children and deeply nested hierarchies.
Also add missing defer s.Close() to existing delete test cases.
Refactor functions that created their own sessions when called from
within existing transactions, which caused "database table is locked"
errors in SQLite's shared-cache mode.
Changes:
- Add files.CreateWithSession() to reuse caller's session
- Refactor DeleteBackgroundFileIfExists() to accept session parameter
- Add variadic session parameter to notifications.Notify() and
Notifiable.ShouldNotify() interface
- Update all Notify callers (~17 sites) to pass their session through
- Use files.CreateWithSession in SaveBackgroundFile and NewAttachment
- Fix test code to commit sessions before assertions