refactor: remove uuid dependency and update checklist/item creation q…

[pikann]

Summary

• Removes the github.com/google/uuid dependency (go.mod / go.sum) — ID generation is now delegated to PostgreSQL.
• Refactors createChecklist and createItem to use INSERT … RETURNING id (via db.Query) instead of generating a UUID client-side.
• Fixes a bug in ChecklistsSection.tsx where the PATCH URL for renaming a checklist was missing the /projects/:projectId prefix, causing the request to hit the wrong route.

Test plan

• Create a checklist and confirm the response contains the database-generated id.
• Create a checklist item and confirm the response contains the database-generated id.
• Rename a checklist and verify the PATCH request is sent to /projects/:projectId/tasks/:taskId/checklists/:id.
• Confirm go.mod no longer lists github.com/google/uuid.