feat: allow test cases in multiple suites

[Sallsup77]

Summary

Adds reusable test case suite memberships so a test case can appear in
multiple test suites while keeping its existing primary test_suite_id for
backward compatibility.

Changes

• Adds a test_case_suite_memberships table and migration.
• Backfills existing test cases as primary suite memberships.
• Adds API endpoints to add/remove secondary suite memberships.
• Updates test case list/count filtering to include secondary suite
  memberships.
• Shows additional suite memberships on the test case detail page.
• Adds edit-page controls for assigning a test case to additional suites and
  optional suite-specific sections.
• Keeps existing primary suite behavior intact.

Validation

• npx tsc --noEmit
• npm run build
• PYTHONPYCACHEPREFIX=/tmp/testmona-pycache python3 -m compileall backend/ app