feat(backend): enforce user-owned sessions

backend/test/routes-settings.test.ts

@@ -48,6 +48,35 @@ test("PUT /settings persists and GET reads it back", async () => {
   }
 });
 
+test("settings are isolated by the authenticated repository principal", async () => {
+  const preferences = new Map<string, any>();
+  const runner = {
+    withPrincipal: (principal: any) => ({
+      preferencesGet: async () => preferences.get(principal.subject) ?? {},
+      preferencesSet: async (next: any) => { preferences.set(principal.subject, next); },
+    }),
+  };
+  const app = buildApp(loadConfig({ AUTH_MODE: "upstream", THT_HARNESS_DIR: "../harness" }), {
+    thtRunner: runner as any,
+    listModels: async () => [],
+  });
+  const headers = (subject: string) => ({
+    "x-thoth-principal-issuer": "portal",
+    "x-thoth-principal-subject": subject,
+    "x-thoth-is-admin": "0",
+  });
+
+  await app.inject({
+    method: "PUT", url: "/settings", headers: headers("alice"),
+    payload: { workspace: "psd", provider: "zai", model: "glm-5.2", thinking: "high" },
+  });
+  const alice = await app.inject({ method: "GET", url: "/settings", headers: headers("alice") });
+  const bob = await app.inject({ method: "GET", url: "/settings", headers: headers("bob") });
+
+  expect(alice.json()).toMatchObject({ workspace: "psd", thinking: "high" });
+  expect(bob.json()).not.toMatchObject({ workspace: "psd", thinking: "high" });
+});
+
 test("PUT /settings rejects an unknown model when a model list is available", async () => {
   const { app, dir } = appWithTmpSettings({}, {
     listModels: async () => [{ provider: "zai", id: "glm-5.2", name: "GLM 5.2", reasoning: true }],