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/src/config.rs

@@ -19,6 +19,14 @@ pub struct AuthConfig {
     /// `POST /admin/users`. Defaults to `true` (open registration)
     /// for backward compatibility.
     pub allow_self_register: bool,
+    /// When `true`, every API path except a small allowlist
+    /// (`/health`, `/auth/config`, `/auth/login`, `/auth/logout`)
+    /// requires a valid session cookie or bearer token — anonymous
+    /// reads are rejected with 401. Self-registration is also
+    /// force-disabled regardless of [`Self::allow_self_register`]
+    /// so a private instance is locked down with a single switch.
+    /// Defaults to `false` (current public behaviour).
+    pub private_mode: bool,
 }
 
 impl Default for AuthConfig {
@@ -33,6 +41,7 @@ impl Default for AuthConfig {
             // defaults.
             rate_limit: crate::auth::rate_limit::RateLimitConfig::default(),
             allow_self_register: true,
+            private_mode: false,
         }
     }
 }
@@ -181,6 +190,7 @@ impl Config {
                     ) as u32,
                 },
                 allow_self_register: env_bool("ALLOW_SELF_REGISTER", true),
+                private_mode: env_bool("PRIVATE_MODE", false),
             },
             upload: UploadConfig {
                 max_request_bytes: env_usize("MAX_REQUEST_BYTES", 200 * 1024 * 1024),
@@ -373,5 +383,37 @@ mod tests {
         let cfg = CrawlerConfig::from_env().expect("from_env");
         assert_eq!(cfg.manga_limit, 0);
     }
+
+    #[test]
+    fn private_mode_env_parses_true() {
+        let _g = ENV_GUARD.lock().unwrap_or_else(|p| p.into_inner());
+        std::env::set_var("PRIVATE_MODE", "true");
+        std::env::set_var("DATABASE_URL", "postgres://test");
+        let cfg = Config::from_env().expect("from_env");
+        std::env::remove_var("PRIVATE_MODE");
+        std::env::remove_var("DATABASE_URL");
+        assert!(cfg.auth.private_mode);
+    }
+
+    #[test]
+    fn private_mode_env_parses_false() {
+        let _g = ENV_GUARD.lock().unwrap_or_else(|p| p.into_inner());
+        std::env::set_var("PRIVATE_MODE", "false");
+        std::env::set_var("DATABASE_URL", "postgres://test");
+        let cfg = Config::from_env().expect("from_env");
+        std::env::remove_var("PRIVATE_MODE");
+        std::env::remove_var("DATABASE_URL");
+        assert!(!cfg.auth.private_mode);
+    }
+
+    #[test]
+    fn private_mode_defaults_to_false() {
+        let _g = ENV_GUARD.lock().unwrap_or_else(|p| p.into_inner());
+        std::env::remove_var("PRIVATE_MODE");
+        std::env::set_var("DATABASE_URL", "postgres://test");
+        let cfg = Config::from_env().expect("from_env");
+        std::env::remove_var("DATABASE_URL");
+        assert!(!cfg.auth.private_mode);
+    }
 }