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/ingress.yaml

@@ -0,0 +1,26 @@
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: mass-driver-ingress
+  namespace: mass-driver
+  annotations:
+    nginx.ingress.kubernetes.io/rewrite-target: /
+spec:
+  ingressClassName: nginx
+  rules:
+    - http:
+        paths:
+          - path: /api
+            pathType: Prefix
+            backend:
+              service:
+                name: backend
+                port:
+                  number: 3001
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: frontend
+                port:
+                  number: 80