feat(core): add label CRUD service

[dylany3]

Issue for this PR

Closes #14

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a Label service (packages/core/src/label.ts) with create, get, list, update, and remove for labels. Built on top of #21 (label table), so the base is brianz2/database-labels until that merges.

Invalid operations fail with typed errors instead of doing something weird:

• Editing or deleting a nonexistent/already deleted label: NotFoundError
• Empty or too-long name (over 64 chars): InvalidNameError
• Duplicate name under the same parent (case-insensitive): DuplicateNameError
• Moving a label under itself or its own child: CycleError

Deleting a label also deletes its child labels and returns all deleted IDs, so #15 can clean up session assignments. Open to feedback on this and on requiring unique sibling names.

How did you verify your code works?

Added packages/core/test/label.test.ts with 18 tests covering each operation and every invalid case above. Run with bun test test/label.test.ts in packages/core; all pass. Typecheck and Prettier also pass.

Screenshots / recordings

N/A, backend only

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[jahuang24]

my claude found a mistake.

Issue: {"parentID": ""} (an empty string) returns a 500 instead of a 404. The service treats "" as "no parent", skips checking that the parent exists, and then inserts parent_id = "", which fails the database's foreign key check.
Possible Solution: Change Label.ID schema to reject empty strings, which would make it a clean 400.

u just got claudemogged

[Yumeseeker]

For create and update, if the parentID is "", it could bypass the requireRow check. maybe make code explicit like if (parentID !== null)...

Shouldn't matter for users since all created labels are validated, but could be an edge case for internal use?

[dylany3]

@jahuang24 @Yumeseeker thanks for catching this! Fixed in ccc8550: create and update now check parentID !== null instead of truthiness, so "" goes through requireRow and returns NotFoundError instead of hitting the DB foreign key. Added a regression test for both create and update (19 tests passing, CI green).