feat(server): add tenant management, health checks, and database export/import APIs

rust/crates/rustdb-auth/src/lib.rs

@@ -282,6 +282,27 @@ impl AuthEngine {
             .collect()
     }
 
+    pub fn list_users(&self) -> Vec<AuthenticatedUser> {
+        let users = self.users.read().unwrap_or_else(|poisoned| poisoned.into_inner());
+        let mut result: Vec<AuthenticatedUser> = users
+            .values()
+            .map(AuthUser::to_authenticated_user)
+            .collect();
+        result.sort_by(|a, b| a.database.cmp(&b.database).then(a.username.cmp(&b.username)));
+        result
+    }
+
+    pub fn drop_users_for_database(&self, database: &str) -> Result<usize, AuthError> {
+        let mut users = self.users.write().unwrap_or_else(|poisoned| poisoned.into_inner());
+        let before = users.len();
+        users.retain(|_, user| user.database != database);
+        let dropped = before.saturating_sub(users.len());
+        if dropped > 0 {
+            self.persist_locked(&users)?;
+        }
+        Ok(dropped)
+    }
+
     pub fn start_scram_sha256(
         &self,
         database: &str,