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>

backend/app/routers/tests.py

@@ -11,6 +11,7 @@ PATCH  /tests/{id}/red               — Red Team updates (draft, red_executing)
 PATCH  /tests/{id}/blue              — Blue Team updates (blue_evaluating)
 POST   /tests/{id}/start-execution   — draft → red_executing
 POST   /tests/{id}/submit-red        — red_executing → blue_evaluating
+POST   /tests/{id}/start-blue-work   — blue tech picks up (sets Tempo timer)
 POST   /tests/{id}/submit-blue       — blue_evaluating → in_review
 POST   /tests/{id}/validate-red      — Red Lead validates
 POST   /tests/{id}/validate-blue     — Blue Lead validates
@@ -62,6 +63,7 @@ from app.services.test_workflow_service import (
     start_execution as wf_start_execution,
     submit_red_evidence as wf_submit_red,
     submit_blue_evidence as wf_submit_blue,
+    start_blue_work as wf_start_blue_work,
     validate_as_red_lead as wf_validate_red,
     validate_as_blue_lead as wf_validate_blue,
     reopen_test as wf_reopen,
@@ -415,6 +417,26 @@ def submit_blue(
     return test
 
 
+# ---------------------------------------------------------------------------
+# POST /tests/{id}/start-blue-work  —  blue tech picks up test for evaluation
+# ---------------------------------------------------------------------------
+
+
+@router.post("/{test_id}/start-blue-work", response_model=TestOut)
+def start_blue_work(
+    test_id: uuid.UUID,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(require_any_role("blue_tech", "blue_lead")),
+):
+    """Blue tech picks up the test to start evaluating. Sets the Tempo timer start."""
+    test = crud_get_test_or_raise(db, test_id)
+    with UnitOfWork(db) as uow:
+        test = wf_start_blue_work(db, test, current_user)
+        uow.commit()
+    db.refresh(test)
+    return test
+
+
 # ---------------------------------------------------------------------------
 # POST /tests/{id}/pause-timer  —  pause the active phase timer
 # ---------------------------------------------------------------------------