[Endpoints] Case result submission + auto-upsert

[bg-playground]

Add per-test-case result endpoints. Auto-upserts new test cases when external_id is supplied but test_case_id is not — letting frameworks "discover" tests in BGSTM without pre-registration.

Parent: #291
Depends on: router skeleton sub

Endpoints

• POST /api/v1/external-results/case
• PATCH /api/v1/external-results/case/{id}
• GET /api/v1/external-results/case/{id}

Auto-upsert rules

• test_case_id supplied + exists: link to it
• test_case_id supplied + missing: 404
• Only external_id supplied: lookup by (project_id, external_id). Found → link. Not found → create with auto_registered=true
• Neither supplied: 422

Traceability auto-link

If requirement_ids: [...] provided, write idempotent entries into the existing traceability table.

Status transition rules

• started → any
• passed/failed/skipped → only flaky (re-classification)
• aborted → terminal
• Violations return 409

Acceptance criteria

• All three endpoints work behind runner token auth
• FK enforced for known test_case_id
• Auto-upsert path creates rows with auto_registered=true
• requirement_ids triggers idempotent inserts into traceability
• Status transition rules enforced + tested
• Migration adds external_case_results table
• Integration tests cover 4 upsert scenarios + 3 transition scenarios

Out of scope

• Artifacts (separate sub)
• Audit log (separate sub)

[bg-playground]

Closing as partially completed. Shipped:

POST /external-results/case with auto-upsert (external_id lookup → create with auto_registered=true)
PATCH /external-results/case/{id}
requirement_ids triggers idempotent traceability links
External-ID resolution path added in #324 (requirement_external_ids → resolves via requirements.external_id)
Status-transition rules enforced
Smoke validates 4 upsert scenarios via external_results.case.create audit entries
Deferred (filed as new issue): GET /external-results/case/{id} — assert.py currently reconstructs case-result rows from audit-log entries because this endpoint isn't on main yet. Filing a small v0.3 follow-up to add it; closing #301 now since the rest of its scope is met.