feat: add PRIVATE_MODE site-wide auth gate (0.48.0)

When `PRIVATE_MODE=true`, every API path except a small allowlist
(`/health`, `/auth/{config,login,logout,register}`) requires a valid
session cookie or bearer token — anonymous reads are rejected with
401. Self-registration is force-disabled in private mode regardless
of `ALLOW_SELF_REGISTER`, so a locked-down instance flips with a
single switch (admins still mint accounts via `POST /admin/users`).

The backend gate is a tower middleware that reuses the existing
`CurrentUser` extractor, so the cookie + bearer paths cannot drift
from per-handler auth. `/auth/config` now exposes the flag plus the
effective `self_register_enabled` value so the frontend can render
the navbar correctly on the first paint.

On the frontend, a new universal root `+layout.ts` fetches the
config and redirects anonymous visitors to `/login?next=<path>`
before page-specific loads fire. The redirect is UX only — the
backend middleware is the source of truth, so crafted requests
still 401.

Defaults stay public (`PRIVATE_MODE=false`); existing deployments
need no env change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/common/mod.rs

@@ -92,6 +92,21 @@ pub fn harness_with_self_register_disabled(pool: PgPool) -> Harness {
     harness_with_auth_config(pool, storage, storage_dir, auth)
 }
 
+/// Like [`harness`] but flips `PRIVATE_MODE` on so the site-wide auth
+/// gate is exercised. `allow_self_register` stays at its default `true`
+/// to verify that private mode force-disables self-registration on top
+/// of whatever `ALLOW_SELF_REGISTER` says.
+pub fn harness_with_private_mode(pool: PgPool) -> Harness {
+    let storage_dir = tempfile::tempdir().expect("tempdir");
+    let storage = Arc::new(LocalStorage::new(storage_dir.path()));
+    let auth = AuthConfig {
+        cookie_secure: false,
+        private_mode: true,
+        ..AuthConfig::default()
+    };
+    harness_with_auth_config(pool, storage, storage_dir, auth)
+}
+
 /// Like [`harness`] but configures a tight auth rate limit. Used by
 /// the brute-force-rate-limiting test.
 pub fn harness_with_auth_rate_limit(