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/pages/TestDetailPage.tsx

@@ -10,6 +10,7 @@ import {
   startExecution,
   submitRedEvidence,
   submitBlueEvidence,
+  startBlueWork,
   validateAsRedLead,
   validateAsBlueLead,
   reopenTest,
@@ -190,6 +191,15 @@ export default function TestDetailPage() {
     onError: (err: unknown) => showToast(extractError(err), "error"),
   });
 
+  const startBlueWorkMutation = useMutation({
+    mutationFn: () => startBlueWork(testId!),
+    onSuccess: () => {
+      invalidateAll();
+      showToast("Blue evaluation started", "success");
+    },
+    onError: (err: unknown) => showToast(extractError(err), "error"),
+  });
+
   const validateRedLeadMutation = useMutation({
     mutationFn: (payload: { red_validation_status: "approved" | "rejected"; red_validation_notes?: string }) =>
       validateAsRedLead(testId!, payload),
@@ -302,6 +312,7 @@ export default function TestDetailPage() {
     startExecMutation.isPending ||
     submitRedMutation.isPending ||
     submitBlueMutation.isPending ||
+    startBlueWorkMutation.isPending ||
     reopenMutation.isPending;
 
   // ── Loading / Error states ─────────────────────────────────────
@@ -370,6 +381,7 @@ export default function TestDetailPage() {
         onStartExecution={() => startExecMutation.mutate()}
         onSubmitRed={() => submitRedMutation.mutate()}
         onSubmitBlue={() => submitBlueMutation.mutate()}
+        onStartBlueWork={() => startBlueWorkMutation.mutate()}
         onOpenValidateModal={(side) => setValidationModal({ open: true, side })}
         onReopen={() => setConfirmReopen(true)}
         onPauseTimer={() => pauseTimerMutation.mutate()}