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>

frontend/src/lib/auth-config.svelte.ts

@@ -16,6 +16,7 @@ import { getAuthConfig } from './api/auth';
 
 class AuthConfigStore {
     self_register_enabled = $state(true);
+    private_mode = $state(false);
     loaded = $state(false);
     private loading = false;
 
@@ -25,6 +26,7 @@ class AuthConfigStore {
         try {
             const cfg = await getAuthConfig();
             this.self_register_enabled = cfg.self_register_enabled;
+            this.private_mode = cfg.private_mode;
             this.loaded = true;
         } catch {
             // Keep optimistic default; next page mount will retry.
@@ -32,6 +34,16 @@ class AuthConfigStore {
             this.loading = false;
         }
     }
+
+    /** Seed from server-rendered layout data so the very first paint
+     *  doesn't flash the loading state. Used by `+layout.ts` /
+     *  `+layout.svelte` on the universal-load path. Safe to call from
+     *  SSR (no `browser` guard) since it touches only reactive state. */
+    seed(cfg: { self_register_enabled: boolean; private_mode: boolean }): void {
+        this.self_register_enabled = cfg.self_register_enabled;
+        this.private_mode = cfg.private_mode;
+        this.loaded = true;
+    }
 }
 
 export const authConfig = new AuthConfigStore();