POST /api/blocks with a nonexistent blockGroupId returned an
unhandled DbUpdateException (HTTP 500) because CreateBlockHandler
never checked the referenced block group before inserting, letting
the SQLite FK constraint raise. Add a BaseError validation (422),
matching the "does not exist" precedent already used elsewhere.
Found via live E2E testing.
* sort block tree views
* fix naming validation for block scheduling
* show deco group name in deco editor
* show block group name in block editor
* show template group name in template editor
* show deco template group name in deco template editor
* fix template rename crash
* fix block rename crash
* fix deco template rename crash
* fix validation in new form layout
* pin mediatr to last oss version
* update dependencies
* cleanup code in core
* cleanup code in ffmpeg
* cleanup code in infra
* cleanup code in scanner
* cleanup code in application
* cleanup main code
* cleanup test code
* solution-wide code cleanup
timothy
