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>

backend/app/config.py

@@ -57,6 +57,9 @@ class Settings(BaseSettings):
     JIRA_DEFAULT_PROJECT: str = ""
     JIRA_ISSUE_TYPE_TEST: str = "Task"       # tests (campaign or standalone)
     JIRA_ISSUE_TYPE_CAMPAIGN: str = "Epic"   # campaigns (under Initiative)
+    # Jira custom field ID for "Start date" — Jira Cloud team-managed: customfield_10015
+    # Override with the correct field ID for your Jira instance if different.
+    JIRA_START_DATE_FIELD: str = "customfield_10015"
 
     # ── Tempo Integration ─────────────────────────────────────────────
     TEMPO_ENABLED: bool = False