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

frontend/src/api/campaigns.ts

@@ -31,6 +31,7 @@ export interface Campaign {
   threat_actor_id: string | null;
   threat_actor_name: string | null;
   created_by: string | null;
+  start_date: string | null;
   scheduled_at: string | null;
   completed_at: string | null;
   target_platform: string | null;
@@ -55,6 +56,7 @@ export interface CampaignSummary {
   threat_actor_name: string | null;
   target_platform: string | null;
   tags: string[];
+  start_date: string | null;
   created_at: string | null;
   test_count: number;
   completion_pct: number;
@@ -63,6 +65,7 @@ export interface CampaignSummary {
 export interface CampaignCreatePayload {
   name: string;
   description?: string;
+  start_date?: string;  // ISO date YYYY-MM-DD — campaign won't activate before this
   type?: string;
   threat_actor_id?: string;
   target_platform?: string;