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>

backend/app/services/campaign_service.py

@@ -7,6 +7,7 @@ threat actors, and progress calculation.
 import logging
 import uuid
 from datetime import datetime
+from typing import Optional
 
 from sqlalchemy.orm import Session
 
@@ -106,6 +107,8 @@ def generate_campaign_from_threat_actor(
     db: Session,
     actor_id: uuid.UUID,
     user: User,
+    *,
+    start_date: Optional[datetime] = None,
 ) -> Campaign:
     """Auto-generate a campaign from a threat actor's uncovered techniques.
 
@@ -146,6 +149,7 @@ def generate_campaign_from_threat_actor(
         status="draft",
         created_by=user.id,
         tags=[actor.name, "auto-generated"],
+        start_date=start_date,
     )
     db.add(campaign)
     db.flush()  # Get campaign.id