test: add RBAC end-to-end test suite under backend/tests/e2e/rbac/

[yochze]

Builds a comprehensive RBAC e2e suite that exercises real users, roles,
and DataSourceMembership grants against the live FastAPI app using the
existing test_client + create_user/login_user/whoami fixture style.

New fixtures (backend/tests/e2e/rbac/conftest.py):

• invite_user_to_org: real invite+accept flow.
• set_member_role: PUT /organizations/{id}/members/{mid}.
• create_sqlite_ds: creates a private/public SQLite data source against
  the existing chinook fixture.
• grant_ds_membership / revoke_ds_membership.
• rbac_principals: admin + plain member + ds_a_member + ds_b_member +
  outsider (in a separate org), plus two private DSs and one public DS.
• Autouse toggle for allow_uninvited_signups + allow_multiple_organizations
  so the outsider-in-separate-org scenario is actually possible.

New test files:

• test_rbac_registry.py: AST-walks every @requires_permission and
  @requires_data_source_access literal and asserts it exists in the
  ROLES_PERMISSIONS source of truth. Also checks admin is a superset
  of member.
• test_rbac_data_sources.py: matrix of list/detail/PUT/DSM operations
  • list/detail invariant + cross-org isolation.
• test_rbac_instructions.py: private vs global creation, owner vs admin
  updates/deletes, cross-org rejection.
• test_rbac_builds.py: view_builds (member+admin) vs create_builds
  (admin-only), list/detail invariant, cross-org build id check.
• test_rbac_entities.py: list/detail invariant (no-ds variant),
  admin-only mutations, cross-org rejection. Includes an xfail that
  documents a pre-existing MissingGreenlet bug in
  EntityService.create_entity when data_source_ids is provided.
• test_rbac_evals.py: every /api/tests/* endpoint is gated by
  manage_tests (admin only); member + outsider are 403.
• test_rbac_role_principals.py: role transitions take effect
  immediately, DSM does not imply org admin, cross-org token +
  X-Organization-Id rejection, revoking org membership revokes
  all access.

Backend fixes surfaced by the registry/entity tests:

1. routes/metadata_resource.py — two endpoints were decorated with
   @requires_permission('read_data_source'), but there has never been
   a permission by that name in MEMBER_PERMISSIONS / ADMIN_PERMISSIONS.
   Both endpoints would have 403'd for every caller, member and admin
   alike. Renamed to the canonical 'view_data_source' (member-accessible),
   matching every other /data_sources read endpoint.

2. services/entity_service.py — routes/entity.py calls
   service.create_entity(..., force_global=...) on both POST /entities
   and POST /entities/global, but the service signature didn't accept
   force_global. Both endpoints 500'd with TypeError before this change.
   Added force_global=False to the signature, matching the same
   (currently no-op) parameter on InstructionService.create_instruction.

Known issue flagged as xfail:

• EntityService.create_entity with data_source_ids hits
  sqlalchemy.exc.MissingGreenlet in the response serializer because
  the post-refresh data_sources relationship lazy-loads in the sync
  thread. Tracked in test_entity_create_with_data_source_ids_does_not_500.

Results under TESTING=true pytest -s -m e2e --db=sqlite backend/tests/e2e/rbac/:
31 passed, 1 xfailed

Regression check: tests/e2e/test_eval.py, test_membership.py,
test_entity.py, test_instruction.py, test_data_source.py all still pass.