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/CampaignsPage.tsx

@@ -54,6 +54,7 @@ export default function CampaignsPage() {
     description: "",
     type: "custom",
     target_platform: "",
+    start_date: "",
   });
 
   const canCreate = user?.role === "admin" || user?.role === "red_lead" || user?.role === "blue_lead";
@@ -73,7 +74,7 @@ export default function CampaignsPage() {
     onSuccess: (campaign) => {
       queryClient.invalidateQueries({ queryKey: ["campaigns"] });
       setShowCreateForm(false);
-      setNewCampaign({ name: "", description: "", type: "custom", target_platform: "" });
+      setNewCampaign({ name: "", description: "", type: "custom", target_platform: "", start_date: "" });
       navigate(`/campaigns/${campaign.id}`);
     },
   });
@@ -224,6 +225,29 @@ export default function CampaignsPage() {
                   </select>
                 </div>
               </div>
+
+              {/* Start date */}
+              <div className="mt-4">
+                <label className="mb-1.5 block text-sm font-medium text-gray-300">
+                  Start date
+                  <span className="ml-2 text-xs font-normal text-gray-500">
+                    (optional — campaign activates automatically on this date)
+                  </span>
+                </label>
+                <input
+                  type="date"
+                  value={newCampaign.start_date}
+                  min={new Date().toISOString().split("T")[0]}
+                  onChange={(e) => setNewCampaign((c) => ({ ...c, start_date: e.target.value }))}
+                  className="w-full rounded-lg border border-gray-700 bg-gray-800 px-3 py-2 text-sm text-gray-300 focus:border-cyan-500 focus:outline-none [color-scheme:dark]"
+                />
+                {newCampaign.start_date && (
+                  <p className="mt-1.5 flex items-center gap-1.5 text-xs text-amber-400">
+                    <span>⏰</span>
+                    Tests won't be queued until {new Date(newCampaign.start_date + "T00:00:00").toLocaleDateString("en-GB", { day: "numeric", month: "long", year: "numeric" })}. Manual activation before that date is blocked.
+                  </p>
+                )}
+              </div>
             </div>
             <div className="mt-6 flex justify-end gap-3">
               <button