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/backend.yaml

@@ -0,0 +1,62 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: backend
+  namespace: mass-driver
+  labels:
+    app: backend
+spec:
+  replicas: 2
+  selector:
+    matchLabels:
+      app: backend
+  template:
+    metadata:
+      labels:
+        app: backend
+    spec:
+      containers:
+        - name: backend
+          image: mass-driver-backend:latest
+          ports:
+            - containerPort: 3001
+          env:
+            - name: DATABASE_URL
+              valueFrom:
+                secretKeyRef:
+                  name: backend-secrets
+                  key: database-url
+            - name: PORT
+              value: "3001"
+          resources:
+            requests:
+              cpu: 100m
+              memory: 64Mi
+            limits:
+              cpu: 500m
+              memory: 256Mi
+          readinessProbe:
+            httpGet:
+              path: /api/health
+              port: 3001
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          livenessProbe:
+            httpGet:
+              path: /api/health
+              port: 3001
+            initialDelaySeconds: 10
+            periodSeconds: 30
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: backend
+  namespace: mass-driver
+spec:
+  selector:
+    app: backend
+  ports:
+    - port: 3001
+      targetPort: 3001
+      protocol: TCP