feat: add Docker, Drone CI, and k8s deployment manifests

- Dockerfile for Python 3.12 FastAPI app
- Drone CI pipeline to build and push to internal registry
- Kubernetes manifests (Deployment, Service, Secret, Namespace)
- ArgoCD Application for GitOps deployment
- Kustomize base configuration

k8s/base/namespace.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: invest-api