feat: Add admin authentication for Settings panel

- Add sftpgo_admins table to auth.sqlite (synced from PostgreSQL admins)
- Add PgAdmin struct + sync_admins() method in sync.rs
- Add fetch_admins() method in pg_client.rs
- Add AdminLoginRequest/Response + admin_login() + verify_admin_token() in auth.rs
- Add POST /api/v2/admin/login + GET /api/v2/admin/verify endpoints in server.rs
- Add AdminLoginModal UI with password input + localStorage token in page.html
- Test password: admin123 (bcrypt hash updated in PostgreSQL admins table)

Architecture:
- Independent admin auth system (matches SFTPGo design)
- Admin sessions stored in-memory (24h validity)
- bcrypt password verification (cost=10)
- localStorage token persistence for UI
- Settings panel requires admin authentication

Files changed:
- data/init_auth_db.sql: +20 lines
- src/sync.rs: +100 lines
- src/pg_client.rs: +50 lines
- src/auth.rs: +60 lines
- src/server.rs: +50 lines
- src/page.html: +70 lines
Total: ~290 lines added

Tested: Admin sync, login, verify, UI modal all working

src/server.rs

@@ -121,6 +121,9 @@ let state = AppState {
         .route("/api/v2/config", get(get_config_handler))
         .route("/api/v2/config/edit", post(edit_config_handler))
         .route("/api/v2/config/validate", get(validate_config_handler))
+        // Admin authentication API endpoints (public)
+        .route("/api/v2/admin/login", post(admin_login_handler))
+        .route("/api/v2/admin/verify", get(admin_verify_handler))
         // Protected endpoints (require auth)
         .route("/api/v2/tree/:user_id", get(get_tree))
         .route("/api/v2/tree/:user_id/node", post(create_node))
@@ -1563,3 +1566,44 @@ async fn validate_config_handler() -> impl IntoResponse {
         }
     }
 }
+
+async fn admin_login_handler(
+    State(state): State<crate::auth::AuthState>,
+    Json(body): Json<crate::auth::AdminLoginRequest>,
+) -> impl IntoResponse {
+    match state.admin_login(&body.username, &body.password) {
+        Some(response) => (StatusCode::OK, Json(response)).into_response(),
+        None => (
+            StatusCode::UNAUTHORIZED,
+            Json(serde_json::json!({"error": "Invalid admin credentials"})),
+        ).into_response(),
+    }
+}
+
+async fn admin_verify_handler(
+    State(state): State<crate::auth::AuthState>,
+    headers: axum::http::HeaderMap,
+) -> impl IntoResponse {
+    let auth_header = headers
+        .get("Authorization")
+        .and_then(|v| v.to_str().ok())
+        .and_then(|v| v.strip_prefix("Bearer "));
+    
+    if let Some(token) = auth_header {
+        if let Some(session) = state.verify_admin_token(token) {
+            return (
+                StatusCode::OK,
+                Json(serde_json::json!({
+                    "ok": true,
+                    "username": session.username,
+                    "expires_at": session.expires_at
+                })),
+            ).into_response();
+        }
+    }
+    
+    (
+        StatusCode::UNAUTHORIZED,
+        Json(serde_json::json!({"ok": false, "error": "Invalid admin token"})),
+    ).into_response()
+}