feat(campaigns): start_date for threat-actor-generated campaigns

Backend:
- campaign_service.generate_campaign_from_threat_actor: accept optional
  start_date kwarg and set it on the Campaign model
- campaigns router: new GenerateFromActorPayload schema, /from-threat-actor
  endpoint now accepts optional body with start_date

Frontend:
- generateCampaignFromThreatActor API: accept optional options param
- Generate Campaign modal: date picker + warning message, same UX as the
  manual create form

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

frontend/src/pages/CampaignsPage.tsx

@@ -49,6 +49,7 @@ export default function CampaignsPage() {
   const [showCreateForm, setShowCreateForm] = useState(false);
   const [showActorSelector, setShowActorSelector] = useState(false);
   const [actorSearch, setActorSearch] = useState("");
+  const [actorStartDate, setActorStartDate] = useState("");
   const [newCampaign, setNewCampaign] = useState({
     name: "",
     description: "",
@@ -87,10 +88,12 @@ export default function CampaignsPage() {
   });
 
   const generateMutation = useMutation({
-    mutationFn: (actorId: string) => generateCampaignFromThreatActor(actorId),
+    mutationFn: (actorId: string) =>
+      generateCampaignFromThreatActor(actorId, actorStartDate ? { start_date: actorStartDate } : undefined),
     onSuccess: (campaign) => {
       queryClient.invalidateQueries({ queryKey: ["campaigns"] });
       setShowActorSelector(false);
+      setActorStartDate("");
       navigate(`/campaigns/${campaign.id}`);
     },
   });
@@ -289,6 +292,29 @@ export default function CampaignsPage() {
               />
             </div>
 
+            {/* Start date */}
+            <div className="mb-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={actorStartDate}
+                min={new Date().toISOString().split("T")[0]}
+                onChange={(e) => setActorStartDate(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]"
+              />
+              {actorStartDate && (
+                <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(actorStartDate + "T00:00:00").toLocaleDateString("en-GB", { day: "numeric", month: "long", year: "numeric" })}
+                </p>
+              )}
+            </div>
+
             {/* Actor list */}
             <div className="max-h-72 overflow-y-auto rounded-lg border border-gray-800">
               {isLoadingActors ? (