DEV: allow program experts to read filter schema

[TThanos3000]

Summary

• add a scoped IsAdminManagerOrExpertOfProgram permission for the program filter schema
• allow experts assigned to the requested program to read GET /programs/<id>/filters/
• preserve manager, staff, and superuser access through the existing can_manage_program policy
• keep /programs/<id>/filters/ read-only and leave project filtering and all other manager endpoints unchanged

Access matrix

• anonymous: 401
• authenticated participant: 403
• expert of another program: 403
• expert of this program: 200
• manager of this program: 200
• staff/superuser: 200

Verification

• targeted program filter tests: 18/18 passed locally
• GitHub Backend PostgreSQL CI targeted tests: 39/39 passed
• GitHub Backend PostgreSQL CI full suite: 580/580 passed
• GitHub standard Tests: 580/580 passed
• GitHub Lint: passed
• scoped Black: passed
• scoped Flake8: passed
• manage.py check: passed
• manage.py check --tag models: passed
• makemigrations --check --dry-run: no changes detected
• git diff --check: passed

No frontend, React, workflows, Docker, deploy, models, migrations, rating semantics, or API response contract changes are included.