fix: remaining security and deployment hardening (#6 #7 #10)

#7 Transport Security:
- Removed legacy _ssl_ctx alias from config.py
- proxy.py now uses _internal_ssl_ctx directly (explicitly scoped)
- No global TLS bypass remains

#10 Deployment Hardening:
- Inventory Dockerfile: non-root (node user), health check, production deps
- Budget Dockerfile: non-root (node user), health check, npm ci, multi-stage ready
- Frontend-v2 Dockerfile: multi-stage build, non-root (node user), health check
- Added /health endpoints to inventory and budget (before auth middleware)
- All 6 containers now run as non-root with health checks

All services verified: gateway, trips, fitness, inventory, budget, frontend

gateway/config.py

@@ -68,5 +68,3 @@ _internal_ssl_ctx = ssl.create_default_context()
 _internal_ssl_ctx.check_hostname = False
 _internal_ssl_ctx.verify_mode = ssl.CERT_NONE
 
-# Legacy alias — to be removed once all callers are updated
-_ssl_ctx = _internal_ssl_ctx