webui: rename auth_store → webui_store

The plugin's SQLite store will own more than accounts (taxonomy, RSS),
so give it a general name. Mechanical rename of files + symbols; default
DB filename is now webui.db. No behavior change.

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

plugins/webui/webui_store.h

@@ -0,0 +1,47 @@
+/* webui_store.h — SQLite-backed persistence for all web-UI-owned state:
+ * accounts, the category/tag taxonomy, and RSS feeds/rules/indexers.
+ *
+ * Owned entirely by the webui plugin (the daemon persists none of this).
+ * Account passwords are PBKDF2-HMAC-SHA256 with a per-user random salt. All
+ * calls are thread-safe (the store serializes access to its SQLite handle). */
+#ifndef NAUT_WEBUI_STORE_H
+#define NAUT_WEBUI_STORE_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <jansson.h>
+
+typedef struct webui_store webui_store;
+
+/* Open (creating if needed) the account database at `path`. Returns NULL on
+ * failure. The schema is created/migrated on open. */
+webui_store *webui_store_open(const char *path);
+void webui_store_close(webui_store *s);
+
+/* Number of accounts, or -1 on error. */
+int webui_store_user_count(webui_store *s);
+/* Number of admin accounts, or -1 on error. */
+int webui_store_admin_count(webui_store *s);
+bool webui_store_user_exists(webui_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 webui_store_verify(webui_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 webui_store_create_user(webui_store *s, const char *username,
+                            const char *password, const char *role);
+
+bool webui_store_set_password(webui_store *s, const char *username,
+                             const char *password);
+/* Change an account's role ("admin"/"user"). */
+bool webui_store_set_role(webui_store *s, const char *username, const char *role);
+bool webui_store_delete_user(webui_store *s, const char *username);
+
+/* Append {username, role, createdAt} objects (sorted by username) to the
+ * json array `out`. Returns false on error. */
+bool webui_store_list_users(webui_store *s, json_t *out);
+
+#endif /* NAUT_WEBUI_STORE_H */