Add upstream caching infrastructure and refactor CI pipeline

Upstream Caching (Epic #68-#75, #105):
- Add upstream_sources and cache_settings tables with migrations
- Add cache management API endpoints (CRUD for sources, settings)
- Add environment variable overrides for upstream sources and cache settings
- Add encryption module for storing credentials securely
- Add frontend Admin Cache Management page
- Add is_system field to projects for system cache distinction
- Add purge_seed_data for transitioning to production-like environments

CI Pipeline Refactoring:
- Remove reset jobs (reset_stage_pre, reset_stage)
- Add ephemeral orchard-test deployment for main branch testing
- Run integration tests on ephemeral deployment before promoting to stage
- Stage is now long-running pre-prod (smoke tests only)
- Disable prosper_setup for tag pipelines

backend/requirements.txt

@@ -11,10 +11,10 @@ python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
 bcrypt==4.0.1
 slowapi==0.1.9
+httpx>=0.25.0
 
 # Test dependencies
 pytest>=7.4.0
 pytest-asyncio>=0.21.0
 pytest-cov>=4.1.0
-httpx>=0.25.0
 moto[s3]>=4.2.0