feat(backend): scaffold Fastify+TS + /health

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/app.ts

@@ -0,0 +1,7 @@
+import Fastify, { type FastifyInstance } from "fastify";
+import type { AppConfig } from "./config.js";
+export function buildApp(_config: AppConfig): FastifyInstance {
+  const app = Fastify({ logger: false });
+  app.get("/health", async () => ({ status: "ok" }));
+  return app;
+}

backend/src/config.ts

@@ -0,0 +1,17 @@
+export interface AppConfig {
+  port: number; harnessDir: string; thtBin: string; piBin: string;
+  authMode: "none" | "mock" | "oidc";
+  defaults: { provider?: string; model?: string; thinking?: string };
+  maxPiProcesses: number;
+}
+export function loadConfig(env: Record<string, string | undefined>): AppConfig {
+  return {
+    port: Number(env.PORT ?? 8787),
+    harnessDir: env.THT_HARNESS_DIR ?? "../harness",
+    thtBin: env.THT_BIN ?? "tht",
+    piBin: env.PI_BIN ?? "pi",
+    authMode: (env.AUTH_MODE as AppConfig["authMode"]) ?? "none",
+    defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
+    maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
+  };
+}

backend/src/server.ts

@@ -0,0 +1,6 @@
+import { buildApp } from "./app.js";
+import { loadConfig } from "./config.js";
+const config = loadConfig(process.env);
+const app = buildApp(config);
+app.listen({ port: config.port, host: "127.0.0.1" })
+  .then((addr) => console.log(`backend listening on ${addr}`));