fix: remove unused uuid dependency and update scenario creation logic

[pikann]

Summary

• Removes the github.com/google/uuid dependency (go.mod / go.sum) — ID generation is now delegated to PostgreSQL.
• Refactors createScenario to use INSERT … RETURNING id (via db.Query) instead of generating a UUID client-side and passing it as $1.
• Fixes a duplicated path segment in plugin.json: /projects/:projectId/projects/:projectId/tasks/… → /projects/:projectId/tasks/….

Test plan

• Create a BDD scenario via the API and confirm the response contains the database-generated id.
• Verify the PATCH route (/projects/:projectId/tasks/:taskId/bdd-scenarios/:scenarioId) resolves correctly after the path fix.
• Confirm go.mod no longer lists github.com/google/uuid.