feat(campaigns): campaign start date — scheduled activation, Jira start_date

DB: migration b047 adds start_date (DateTime nullable) + index to campaigns.

Backend:
- Campaign model: start_date field
- CampaignCreate/Update schemas: accept start_date (ISO string)
- CRUD service: persist + serialize start_date in both serializers
- Activation endpoint: blocks manual activation if start_date is in the future
  (campaign will auto-activate via scheduler)
- Scheduler: new hourly job _run_scheduled_campaign_activation — finds draft
  campaigns with start_date <= now, activates them, creates Jira tickets,
  notifies red_tech team
- Jira: campaign + test tickets now include JIRA_START_DATE_FIELD (configurable,
  default customfield_10015). Campaign uses start_date if set, else created_at.
  Tests inherit campaign start_date.
- config.py: JIRA_START_DATE_FIELD setting

Frontend:
- Campaign type: start_date field on Campaign + CampaignSummary
- CampaignCreatePayload: start_date optional field
- Create form: date picker with min=today, warning message explaining behavior
- Campaign detail header: start_date badge showing days remaining or started date

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

frontend/src/pages/CampaignDetailPage.tsx

@@ -254,6 +254,24 @@ export default function CampaignDetailPage() {
                   <Calendar className="h-3.5 w-3.5" />
                   Created {formatDate(campaign.created_at)}
                 </span>
+                {campaign.start_date && (() => {
+                  const sd = new Date(campaign.start_date);
+                  const now = new Date();
+                  const isPast = sd <= now;
+                  const diffDays = Math.ceil((sd.getTime() - now.getTime()) / 86400000);
+                  return (
+                    <span className={`flex items-center gap-1.5 rounded-full border px-2.5 py-0.5 text-xs font-medium ${
+                      isPast
+                        ? "border-green-500/30 bg-green-500/10 text-green-400"
+                        : "border-amber-500/30 bg-amber-500/10 text-amber-400"
+                    }`}>
+                      <Clock className="h-3 w-3" />
+                      {isPast
+                        ? `Started ${formatDate(campaign.start_date)}`
+                        : `Starts ${formatDate(campaign.start_date)} (${diffDays}d)`}
+                    </span>
+                  );
+                })()}
                 {campaign.completed_at && (
                   <span className="flex items-center gap-1 text-green-400">
                     <CheckCircle className="h-3.5 w-3.5" />