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/components/test-detail/TestDetailHeader.tsx

@@ -51,6 +51,7 @@ interface TestDetailHeaderProps {
   onStartExecution: () => void;
   onSubmitRed: () => void;
   onSubmitBlue: () => void;
+  onStartBlueWork: () => void;
   onOpenValidateModal: (side: "red" | "blue") => void;
   onReopen: () => void;
   onPauseTimer: () => void;
@@ -67,6 +68,7 @@ export default function TestDetailHeader({
   onStartExecution,
   onSubmitRed,
   onSubmitBlue,
+  onStartBlueWork,
   onOpenValidateModal,
   onReopen,
   onPauseTimer,
@@ -127,22 +129,38 @@ export default function TestDetailHeader({
       );
     }
 
-    // Blue Team in blue_evaluating -> Submit for Review
+    // Blue Team in blue_evaluating:
+    //   - if not picked up yet: show "Start Evaluation" button
+    //   - if already picked up: show "Submit for Review" button
     if (
       test.state === "blue_evaluating" &&
       (role === "blue_tech" || role === "blue_lead" || role === "admin")
     ) {
-      buttons.push(
-        <button
-          key="submit-blue"
-          onClick={onSubmitBlue}
-          disabled={isTransitioning}
-          className="flex items-center gap-1.5 rounded-lg bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-500 transition-colors disabled:opacity-50"
-        >
-          {isTransitioning ? <Loader2 className="h-4 w-4 animate-spin" /> : <Send className="h-4 w-4" />}
-          Submit for Review
-        </button>,
-      );
+      if (!test.blue_work_started_at) {
+        buttons.push(
+          <button
+            key="start-blue-work"
+            onClick={onStartBlueWork}
+            disabled={isTransitioning}
+            className="flex items-center gap-1.5 rounded-lg bg-indigo-600 px-4 py-2 text-sm font-medium text-white hover:bg-indigo-500 transition-colors disabled:opacity-50"
+          >
+            {isTransitioning ? <Loader2 className="h-4 w-4 animate-spin" /> : <Play className="h-4 w-4" />}
+            Start Evaluation
+          </button>,
+        );
+      } else {
+        buttons.push(
+          <button
+            key="submit-blue"
+            onClick={onSubmitBlue}
+            disabled={isTransitioning}
+            className="flex items-center gap-1.5 rounded-lg bg-blue-600 px-4 py-2 text-sm font-medium text-white hover:bg-blue-500 transition-colors disabled:opacity-50"
+          >
+            {isTransitioning ? <Loader2 className="h-4 w-4 animate-spin" /> : <Send className="h-4 w-4" />}
+            Submit for Review
+          </button>,
+        );
+      }
     }
 
     // Red Lead in in_review -> Validate Red
@@ -264,10 +282,10 @@ export default function TestDetailHeader({
         />
       );
     }
-    if (test.state === "blue_evaluating" && test.blue_started_at) {
+    if (test.state === "blue_evaluating" && test.blue_work_started_at) {
       return (
         <LiveTimer
-          startedAt={test.blue_started_at}
+          startedAt={test.blue_work_started_at}
           pausedAt={test.paused_at}
           pausedSeconds={test.blue_paused_seconds}
           label="Blue Team Timer"