test: add tweet creation + comment tests (13 tests)

Covers: create, blank content validation, guest restriction, read,
owner edit, non-owner edit forbidden, owner delete, non-owner delete
forbidden, reply creation, comment_count aggregate, tweet owner
deletes comment, third party forbidden, guest comment forbidden.

Key learnings:
- Tweet :destroy is not primary; use Ash.Changeset.for_destroy(:destroy)
- relate_actor fails with Invalid (not Forbidden) when no actor
- Ash.get returns NotFound error on miss; pass not_found_error?: false for nil

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

fix_plan.md

@@ -17,7 +17,9 @@
 - [ ] No CHECK constraint on `likes >= 0` at DB level (low priority, app logic prevents it)
 - [ ] `read :following_feed` — nil actor returns empty list (not a bug)
 - [ ] No search for users or tweets
-- [ ] Missing test coverage: comments, tweet creation, auth flows
+- [x] Tweet creation, update, delete, comment tests (13 tests)
+- [ ] Missing test coverage: auth flows
+- [ ] No pagination on user list (`/users`)
 
 ## Notes