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)

backend/alembic/versions/b045_blue_work_started_at.py

@@ -0,0 +1,16 @@
+"""Add blue_work_started_at to tests table."""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "b045"
+down_revision = "b044"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("tests", sa.Column("blue_work_started_at", sa.DateTime(), nullable=True))
+
+
+def downgrade():
+    op.drop_column("tests", "blue_work_started_at")