feat(tempo): blue team Tempo time from pick-up, not queue entry

Previously blue_started_at was set when the RED team submitted evidence
(= queue open time), so Tempo was getting total queue wait time instead
of actual work time.

Changes:
- DB: add blue_work_started_at column (migration b045), set when a blue
  tech explicitly picks up the test (mirrors red_started_at for red team)
- Workflow: new start_blue_work() function + POST /tests/{id}/start-blue-work
  endpoint (blue_tech / blue_lead roles). Cannot be called twice.
- submit_blue_evidence: uses blue_work_started_at (when available) as the
  phase start for the Tempo worklog, falls back to blue_started_at
- reopen_test: clears blue_work_started_at alongside other timing fields
- Tempo: both red_team_execution and blue_team_evaluation now synced;
  correct work_date and description per activity type
- Frontend: "Start Evaluation" button shown in blue_evaluating state when
  blue_work_started_at is null; live timer shows from pick-up time

What each timestamp tracks:
  blue_started_at      = queue entry (SLA / internal tracking)
  blue_work_started_at = pick-up by blue tech (Tempo start)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/tests.ts

@@ -172,6 +172,12 @@ export async function submitBlueEvidence(testId: string): Promise<Test> {
   return data;
 }
 
+/** Blue tech picks up the test to start evaluating — sets the Tempo timer start. */
+export async function startBlueWork(testId: string): Promise<Test> {
+  const { data } = await client.post<Test>(`/tests/${testId}/start-blue-work`);
+  return data;
+}
+
 // ── Lead Validation ────────────────────────────────────────────────
 
 /** Red Lead approves/rejects the red side. */