fix: scope pgy rollover reads by academic year

[Euda1mon1a]

Summary

• add a shared AY-scoped resident PGY resolver with safe fallback to legacy Person.pgy_level
• switch AcademicBlockService resident rows and AnticipatedLeaveService intern selection to use AY-scoped PGY data and exclude AY-graduated residents
• add focused backend coverage for AY override/fallback behavior and anticipated-leave regeneration within a target AY

Validation

• cd backend && .venv/bin/ruff check app/services/person_academic_year_resolver.py app/services/academic_block_service.py app/services/anticipated_leave_service.py tests/services/test_academic_block_service.py tests/services/test_anticipated_leave_service.py
• cd backend && .venv/bin/pytest tests/services/test_academic_block_service.py -k "get_residents_all or get_residents_filter_by_pgy or get_residents_prefers_academic_year_pgy or get_residents_excludes_academic_year_graduates"
• cd backend && .venv/bin/pytest tests/services/test_anticipated_leave_service.py

Notes

• Local dev DB is already at Alembic head (20260321_program_policy_snapshots); this batch updates app reads, not migrations.
• The broader backend/tests/services/test_academic_block_service.py suite still has pre-existing drift against current models/validator contracts. I logged that in docs/planning/TECHNICAL_DEBT.md as DEBT-031; the focused resident-path tests added here are green.

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