feat(prospecting): add ProspectSecurityAudit model (Phase 1 foundation)

- New model: ProspectSecurityAudit with score, grade, findings_json,
  severity counts, has_https, has_valid_ssl, missing_headers, exposed
  files, technologies, scan_error
- Add last_security_audit_at timestamp to Prospect model
- Add security_audit 1:1 relationship on Prospect

Part of Phase 1: Security Audit in Enrichment Pipeline. Service,
constants, migration, endpoints, and frontend to follow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/prospecting/models/__init__.py

@@ -22,6 +22,7 @@ from app.modules.prospecting.models.prospect import (
 from app.modules.prospecting.models.prospect_contact import ContactType, ProspectContact
 from app.modules.prospecting.models.prospect_score import ProspectScore
 from app.modules.prospecting.models.scan_job import JobStatus, JobType, ProspectScanJob
+from app.modules.prospecting.models.security_audit import ProspectSecurityAudit
 from app.modules.prospecting.models.tech_profile import ProspectTechProfile
 
 __all__ = [
@@ -44,4 +45,5 @@ __all__ = [
     "CampaignChannel",
     "CampaignSendStatus",
     "LeadType",
+    "ProspectSecurityAudit",
 ]