webui: real multi-user accounts (SQLite + PBKDF2)

Replace the single env/generated password with a proper account system,
owned entirely by the webui plugin:

- auth_store: SQLite users table, PBKDF2-HMAC-SHA256 password hashing
  (per-user salt, 210k iterations) via OpenSSL. DB at NAUT_WEBUI_DB or
  an XDG default. Thread-safe (serialized connection).
- Login verifies against the DB; sessions now carry the username + role.
  First run bootstraps an admin from NAUT_AUTH_USER/PASSWORD or a
  generated password (logged once).
- Admin-only user management: GET/POST /api/users, /api/users/delete,
  /api/users/password, /api/users/role. Self-service POST
  /api/account/password. Guards the last admin and invalidates a user's
  sessions on delete or password reset.
- /api/auth/status and /api/login now return the role.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/webui/auth_store.h

@@ -0,0 +1,46 @@
+/* auth_store.h — SQLite-backed user account store for the web UI.
+ *
+ * Owned entirely by the webui plugin. Passwords are stored as PBKDF2-HMAC-
+ * SHA256 hashes with a per-user random salt. All calls are thread-safe (the
+ * store serializes access to its single SQLite connection internally). */
+#ifndef NAUT_WEBUI_AUTH_STORE_H
+#define NAUT_WEBUI_AUTH_STORE_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <jansson.h>
+
+typedef struct auth_store auth_store;
+
+/* Open (creating if needed) the account database at `path`. Returns NULL on
+ * failure. The schema is created/migrated on open. */
+auth_store *auth_store_open(const char *path);
+void auth_store_close(auth_store *s);
+
+/* Number of accounts, or -1 on error. */
+int auth_store_user_count(auth_store *s);
+/* Number of admin accounts, or -1 on error. */
+int auth_store_admin_count(auth_store *s);
+bool auth_store_user_exists(auth_store *s, const char *username);
+
+/* Verify a username/password pair (constant-time). On success, copies the
+ * account's role ("admin"/"user") into role_out. */
+bool auth_store_verify(auth_store *s, const char *username,
+                       const char *password, char *role_out, size_t role_sz);
+
+/* Create an account. `role` must be "admin" or "user" (defaults to "user" if
+ * NULL/invalid). Returns false if the username already exists or on error. */
+bool auth_store_create_user(auth_store *s, const char *username,
+                            const char *password, const char *role);
+
+bool auth_store_set_password(auth_store *s, const char *username,
+                             const char *password);
+/* Change an account's role ("admin"/"user"). */
+bool auth_store_set_role(auth_store *s, const char *username, const char *role);
+bool auth_store_delete_user(auth_store *s, const char *username);
+
+/* Append {username, role, createdAt} objects (sorted by username) to the
+ * json array `out`. Returns false on error. */
+bool auth_store_list_users(auth_store *s, json_t *out);
+
+#endif /* NAUT_WEBUI_AUTH_STORE_H */