feat(mcp): McpQueryService, read-only SQL core for MCP tools

lib/core/mcp/mcp_access_store.dart

@@ -0,0 +1,43 @@
+import 'dart:convert';
+
+import 'package:querya_desktop/core/storage/local_db.dart';
+
+/// Which connections MCP clients may read. Opt-in per connection: a
+/// connection that was never enabled is invisible to every MCP tool.
+abstract class McpAccessPolicy {
+  Future<bool> canRead(ConnectionRow row);
+}
+
+/// [McpAccessPolicy] stored in `app_settings` as a JSON list of connection ids.
+class McpAccessStore implements McpAccessPolicy {
+  McpAccessStore._();
+
+  static final McpAccessStore instance = McpAccessStore._();
+
+  static const settingsKey = 'mcp_read_connection_ids';
+
+  Future<Set<int>> readableIds() async {
+    final raw = await LocalDb.instance.getAppSetting(settingsKey);
+    if (raw == null || raw.isEmpty) return {};
+    try {
+      final decoded = jsonDecode(raw);
+      if (decoded is List) return {for (final v in decoded) if (v is int) v};
+    } catch (_) {}
+    return {};
+  }
+
+  Future<void> setReadable(int connectionId, bool readable) async {
+    final ids = await readableIds();
+    if (readable) {
+      ids.add(connectionId);
+    } else {
+      ids.remove(connectionId);
+    }
+    await LocalDb.instance
+        .setAppSetting(settingsKey, jsonEncode(ids.toList()..sort()));
+  }
+
+  @override
+  Future<bool> canRead(ConnectionRow row) async =>
+      row.id != null && (await readableIds()).contains(row.id);
+}