feat(placement-cell): Rebuild Placement Cell backend with clean REST API

[vikrantwiz02]

Summary

• Complete rewrite of the placement_cell Django app — replaces the legacy 5810-line monolithic views.py + 25-model schema with a clean service/selector/API architecture
• Deletes all old forms, template views, and URLs; adds a DRF token-authenticated REST API
• CPI is never stored — always fetched live from examination.api.views.calculate_cpi_for_student() against published ResultAnnouncement records

Architecture

placement_cell/
├── models.py — Company, JobPost, PlacementSchedule, PlacementApplication,
│ PlacementResult, StudentPlacementProfile, PlacementAnnouncement,
│ PlacementStatistics, OffCampusPlacement
├── services.py — write operations only (create/update/delete, apply, withdraw,
│ bulk status, upsert profile, off-campus CRUD)
├── selectors.py — read queries only (student CPI, eligible jobs, applicant lists,
│ live batch statistics)
├── api/views.py — thin views: authenticate → role check → service/selector → serialize
├── api/serializers.py
├── api/urls.py
├── tests/test_module.py — 9 service + API tests
└── migrations/ — 0001_initial, 0002 (OffCampusPlacement + PPO type), 0003 (apply_link)

Key design decisions

• Live CPI: selectors.get_student_published_cpi() calls calculate_cpi_for_student() — no stored CPI field on profile
• Opted-out is one-way: services.upsert_student_profile() ignores opted_out=False if already True
• Off-campus = placed: services.add_offcampus_placement() sets StudentPlacementProfile.is_placed = True
• Live statistics: _compute_batch_live_stats() queries live from Student/Application/OffCampus/Result tables — no stale snapshot dependency
• apply_link tracking: student clicking an external apply link still hits POST /stu/jobs/<id>/apply/ to record the application in DB before opening the external URL

API surface

Prefix	Roles	Endpoints
/placement-cell/api/stu/	student	dashboard, jobs, apply, withdraw, applications, profile
/placement-cell/api/officer/	placement officer	companies, jobs, toggle, applicants, status, bulk-status, students, update, offcampus, announcements, statistics, export
/placement-cell/api/chairman/	placement chairman	statistics, students, export
/placement-cell/api/dean/	dean + faculty	statistics, announcements, batches

Test plan

• python manage.py check → 0 errors
• python manage.py migrate → clean
• GET /placement-cell/api/stu/dashboard/ with student token → 200
• POST /placement-cell/api/stu/jobs/<id>/apply/ with CPI below min_cpi → 400 with message
• GET /placement-cell/api/officer/jobs/ with student token → 403
• GET /placement-cell/api/officer/export/ → valid XLSX download
• python manage.py test placement_cell → all 9 tests pass
• Verify globals, otheracademic, academic_procedures still import cleanly