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>

crates/mass-driver-backend/src/main.rs

@@ -1,3 +1,49 @@
-fn main() {
-    println!("mass-driver backend — not yet implemented");
+mod db;
+mod routes;
+
+use axum::{routing::{get, post}, Router};
+use sqlx::postgres::PgPoolOptions;
+use tower_http::{compression::CompressionLayer, cors::CorsLayer};
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt::init();
+
+    let database_url =
+        std::env::var("DATABASE_URL").expect("DATABASE_URL environment variable must be set");
+    let port = std::env::var("PORT").unwrap_or_else(|_| "3001".to_string());
+
+    tracing::info!("Connecting to database...");
+    let pool = PgPoolOptions::new()
+        .max_connections(10)
+        .connect(&database_url)
+        .await
+        .expect("Failed to connect to Postgres");
+
+    db::init_db(&pool).await.expect("Failed to initialize database tables");
+    tracing::info!("Database initialized");
+
+    let app = Router::new()
+        .route("/api/health", get(routes::health))
+        .route("/api/cache/transfer-matrix", post(routes::store_matrix))
+        .route(
+            "/api/cache/transfer-matrix/{config_hash}",
+            get(routes::get_matrix),
+        )
+        .route("/api/cache/route", post(routes::store_route))
+        .route("/api/cache/route", get(routes::get_route))
+        .layer(CompressionLayer::new())
+        .layer(CorsLayer::permissive())
+        .with_state(pool);
+
+    let addr = format!("0.0.0.0:{port}");
+    tracing::info!("Listening on {addr}");
+
+    let listener = tokio::net::TcpListener::bind(&addr)
+        .await
+        .expect("Failed to bind listener");
+
+    axum::serve(listener, app)
+        .await
+        .expect("Server error");
 }