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/app.rs

@@ -3,8 +3,10 @@ use std::sync::atomic::AtomicBool;
 
 use anyhow::Context;
 use async_trait::async_trait;
-use axum::extract::DefaultBodyLimit;
+use axum::extract::{DefaultBodyLimit, FromRequestParts, Request, State};
 use axum::http::{HeaderName, HeaderValue, Method};
+use axum::middleware::{self, Next};
+use axum::response::Response;
 use axum::Router;
 use sqlx::postgres::PgPoolOptions;
 use sqlx::PgPool;
@@ -12,7 +14,9 @@ use tokio_util::sync::CancellationToken;
 use tower_http::cors::{AllowOrigin, CorsLayer};
 use tower_http::trace::TraceLayer;
 
+use crate::auth::extractor::CurrentUser;
 use crate::auth::rate_limit::AuthRateLimiter;
+use crate::error::AppError;
 use crate::config::{AuthConfig, Config, CrawlerConfig, UploadConfig};
 use crate::crawler::browser_manager::{self, BrowserManager};
 use crate::crawler::content::{self, SyncOutcome};
@@ -353,11 +357,62 @@ pub fn router(state: AppState) -> Router {
     let max_request_bytes = state.upload.max_request_bytes;
     Router::new()
         .nest("/api/v1", crate::api::routes())
+        .layer(middleware::from_fn_with_state(
+            state.clone(),
+            private_mode_guard,
+        ))
         .layer(DefaultBodyLimit::max(max_request_bytes))
         .with_state(state)
         .layer(TraceLayer::new_for_http())
 }
 
+/// Paths reachable anonymously even when `PRIVATE_MODE=true`. Login and
+/// logout are needed for the auth flow itself; `/health` is reserved
+/// for load-balancer probes; `/auth/config` lets the frontend decide
+/// whether to render the login form or its anonymous alternatives;
+/// `/auth/register` is exempted from the gate so the handler can
+/// return its informative `registration_disabled` 403 (the same code
+/// public-mode deployments use when `ALLOW_SELF_REGISTER=false`) —
+/// the handler itself force-blocks the request body in private mode,
+/// so no account ever gets created here. Everything else demands a
+/// valid session cookie or bearer token.
+fn is_public_in_private_mode(path: &str) -> bool {
+    matches!(
+        path,
+        "/api/v1/health"
+            | "/api/v1/auth/config"
+            | "/api/v1/auth/login"
+            | "/api/v1/auth/logout"
+            | "/api/v1/auth/register"
+    )
+}
+
+/// Site-wide auth gate for `PRIVATE_MODE=true`. With the flag off this
+/// is a no-op pass-through, so public deployments take no extra DB
+/// hit. With it on, the guard reuses [`CurrentUser`] — the same
+/// session-cookie-then-bearer-token logic the per-handler extractor
+/// uses — so the two paths can never drift.
+async fn private_mode_guard(
+    State(state): State<AppState>,
+    req: Request,
+    next: Next,
+) -> Result<Response, AppError> {
+    if !state.auth.private_mode {
+        return Ok(next.run(req).await);
+    }
+    if is_public_in_private_mode(req.uri().path()) {
+        return Ok(next.run(req).await);
+    }
+    let (mut parts, body) = req.into_parts();
+    match CurrentUser::from_request_parts(&mut parts, &state).await {
+        Ok(_) => {
+            let req = Request::from_parts(parts, body);
+            Ok(next.run(req).await)
+        }
+        Err(_) => Err(AppError::Unauthenticated),
+    }
+}
+
 pub(crate) fn cors_layer(allowed_origins: &[String]) -> CorsLayer {
     if allowed_origins.is_empty() {
         // Same-origin only — no CORS headers emitted.