Add Axum backend, Dockerfiles, and K8s deployment manifests

- Axum backend server: health check, transfer matrix cache (base64 BYTEA),
  route cache (JSONB), CORS, gzip compression, tracing
- Postgres schema: transfer_matrices + cached_routes tables with upserts
- Dockerfile.frontend: 3-stage (wasm-pack → SvelteKit → nginx:alpine)
- Dockerfile.backend: 2-stage (rust build → debian:bookworm-slim)
- nginx.conf: SPA fallback, WASM mime type, /api proxy to backend
- docker-compose.yml: Postgres + backend for local development
- K8s manifests: namespace, frontend/backend deployments with services,
  ingress routing, health probes, secret-based DATABASE_URL

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

k8s/namespace.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: mass-driver