feat(backend): pluggable auth (none/mock/oidc seam)

- authPreHandler(mode) returns Fastify preHandler that sets req.user={id}
- none: uses dev@local
- mock: reads x-mock-user header
- oidc: MVP stub returns 501 + throws
- getUser(req) returns user object

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/test/auth.test.ts

@@ -0,0 +1,24 @@
+import { test, expect } from "vitest";
+import Fastify from "fastify";
+import { authPreHandler, getUser } from "../src/auth/auth.js";
+
+test("mode none assegna dev@local", async () => {
+  const app = Fastify();
+  app.addHook("preHandler", authPreHandler("none"));
+  app.get("/me", async (req) => getUser(req));
+  expect((await app.inject({ method: "GET", url: "/me" })).json()).toEqual({
+    id: "dev@local",
+  });
+});
+
+test("mode mock legge l'header", async () => {
+  const app = Fastify();
+  app.addHook("preHandler", authPreHandler("mock"));
+  app.get("/me", async (req) => getUser(req));
+  const res = await app.inject({
+    method: "GET",
+    url: "/me",
+    headers: { "x-mock-user": "alice" },
+  });
+  expect(res.json()).toEqual({ id: "alice" });
+});