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/src/auth/auth.ts

@@ -0,0 +1,20 @@
+import type { FastifyRequest, FastifyReply } from "fastify";
+
+export function authPreHandler(mode: "none" | "mock" | "oidc") {
+  return async (req: FastifyRequest, reply: FastifyReply) => {
+    if (mode === "none") {
+      (req as any).user = { id: "dev@local" };
+    } else if (mode === "mock") {
+      (req as any).user = {
+        id: (req.headers["x-mock-user"] as string) ?? "mock",
+      };
+    } else {
+      reply.code(501);
+      throw new Error("OIDC non configurato (MVP: usa none/mock)");
+    }
+  };
+}
+
+export function getUser(req: FastifyRequest): { id: string } {
+  return (req as any).user ?? { id: "dev@local" };
+}