feat: remove app name and logo customization feature

Users can no longer customize the app name or logo. The branding API
endpoints, permission, frontend UI, env vars (APP_NAME, MAX_LOGO_SIZE_KB),
and all related tests are removed. Includes a migration to clean up
branding data from existing databases.

tests/e2e/rbac.spec.ts

@@ -174,7 +174,7 @@ base.describe("RBAC - User sees restricted tabs", () => {
     const settingsRes = await fetch(`${API}/api/v1/settings`, {
       method: "PUT",
       headers: authJson(bearerToken),
-      body: JSON.stringify({ appName: "hacked" }),
+      body: JSON.stringify({ testSetting: "hacked" }),
     });
     expect(settingsRes.status).toBe(403);
   });
@@ -280,7 +280,7 @@ base.describe("RBAC - Editor sees collaborative tabs", () => {
     const settingsRes = await fetch(`${API}/api/v1/settings`, {
       method: "PUT",
       headers: authJson(token as string),
-      body: JSON.stringify({ appName: "hacked" }),
+      body: JSON.stringify({ testSetting: "hacked" }),
     });
     expect(settingsRes.status).toBe(403);
   });