feat: intern batch self-selection endpoint and service

Description

Allow interns to self-select their batch on first login via a one-time onboarding screen. Previously, users.batch_id was always NULL after Corpus login, forcing admins to manually assign every intern. Now interns pick their batch immediately after logging in for the first time.

Related Issue

Closes #1

Type of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Changes Made

Backend (intern-activity-tracker)

File	Change
app/routers/me.py	Added PATCH /api/me/batch endpoint for one-time batch self-assignment
app/routers/batches.py	Relaxed GET /api/batches/ to allow interns to list batches (returns id + name only; count fields are null)
src/.../services/roster_service.py	Added assign_user_batch() — enforces one-time-only batch assignment
app/schemas/roster.py	Changed BatchResponse count fields from int = 0 to Optional[int] = None
tests/test_roster_service.py	4 new tests: success, already-set, nonexistent-batch, nonexistent-user

API Contract

PATCH /api/me/batch

Request:

{ "batch_id": 1 }

Responses:

• 200 — Updated profile with batch_id set
• 400 — Batch not found or does not exist
• 409 — Batch already set (one-time choice only)

How It Works

1. Intern logs in via Corpus → batch_id is NULL
2. Frontend calls GET /api/batches/ → returns batch id + name (counts are null)
3. Intern picks a batch → frontend calls PATCH /api/me/batch
4. Backend validates: batch exists? not already set? → updates users.batch_id
5. Intern is redirected to the main app with their batch set

How Has This Been Tested?

• Unit tests pass: 4 new tests, 20 total in test_roster_service.py
• ruff check . — All checks passed
• mypy — Success: no issues found in 51 source files
• Manual testing in Swagger UI (/docs)

Screenshots (if appropriate)

N/A — backend-only changes. See companion MR in team_standup_tracker for frontend screenshots.

Checklist

• My code follows the project's coding conventions
• I have performed a self-review of my own code
• I have commented my code where necessary
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix/feature works
• New and existing unit tests pass locally

Companion MR

• Frontend: team_standup_tracker → MR title: feat: intern batch self-selection onboarding screen
• Branch: feat/intern-batch-self-selection in both repos

Semantic Versioning Impact

• Patch
• Minor
• Major

Reviewer Notes

• The assign_user_batch() service function has a double-guard: the endpoint checks batch_id before calling the service, and the service checks again inside a transaction. This prevents race conditions from concurrent requests.
• BatchResponse count fields (team_count, member_count) were changed from int = 0 to Optional[int] = None. This means the frontend should handle null values for interns. Mentors/admins will still receive actual integers.