feat: remove MongoDB legacy code

[remyluslosius]

Summary

• Delete all MongoDB repository classes (10 repositories: base, compliance, enhanced, health, intelligence, plugin_models, plugin, remediation_job, remediation, scan)
• Delete MongoDB models (mongo_models.py, enhanced_mongo_models.py) and clean MongoDB imports from remaining models
• Delete MongoDB-dependent services (compliance_rules/ package, content/import_/ package, content/transformation/transformer.py, mongo_integration_service.py, scan_template_service.py, health_monitoring_tasks.py)
• Delete MongoDB-dependent CLI tools (load_compliance_rules.py, migrate_oval_references.py) and routes (scans/config.py)
• Clean all MongoDB imports (motor, pymongo, beanie, bson) from 12+ remaining service files across plugins/, framework/, rules/, engine/, routes/
• Convert Beanie Document models to Pydantic BaseModel in all plugin service model files
• Stub repository-dependent methods with logger.warning() and sensible defaults (empty lists, zero counts, None)
• Replace MongoDB queries with in-memory storage for orchestration jobs and governance audit events
• Update health monitoring to PostgreSQL-only checks
• Remove MongoDB packages (motor, beanie, pymongo, bson) from requirements.txt

80 files changed, 611 insertions(+), 19,488 deletions(-)

Context

MongoDB was deprecated following Aegis integration (see docs/plans/MONGODB_DEPRECATION_PLAN.md). MongoDB containers were already removed from CI and docker-compose. This PR removes all remaining MongoDB code from backend/app/, completing the deprecation.

Aegis YAML rules replace the MongoDB compliance_rules collection. PostgreSQL replaces all other MongoDB collections. Plugin services that depended on MongoDB repositories now log warnings and return safe defaults until they are reimplemented against PostgreSQL.

Test plan

• All pre-commit hooks pass (black, isort, flake8, mypy, bandit, secrets)
• py_compile passes on all modified files
• Zero remaining from app.repositories imports (except empty __init__.py)
• Zero remaining motor/pymongo/beanie/bson imports
• No direct Beanie ODM calls (.find(), .to_list(), etc.)
• CI pipeline passes
• Backend starts successfully in Docker

🤖 Generated with Claude Code