feat: Initial MVP with home server identity model

- User model with Ed25519 keypair identity
- Register/login on home server (creates keypair)
- Join guest servers with signature verification
- Public endpoints for cross-server identity
- Channel and message CRUD
- JWT authentication middleware

Architecture: Home server model, no federation, client-driven sync

lib/src/utils/extensions.dart

@@ -0,0 +1,25 @@
+import 'dart:convert';
+
+import 'package:relic/relic.dart';
+
+import '../config/config.dart';
+
+extension RequestHelper on Request {
+  /// Parses the request body as JSON
+  Future<Map<String, dynamic>> asJson() async {
+    final bodyText = await readAsString();
+    return jsonDecode(bodyText);
+  }
+}
+
+extension MapToBody on Map<String, dynamic> {
+  /// Converts a Map to a JSON Body
+  Body get toBody => Body.fromString(jsonEncode(this));
+}
+
+extension BodyHelper on Body {
+  /// Creates a JSON body from a Map
+  static Body fromJson(Map<String, dynamic> json) {
+    return Body.fromString(jsonEncode(json));
+  }
+}