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/handlers/public_handlers.dart

@@ -0,0 +1,85 @@
+import 'package:chat_server/models.dart';
+import 'package:chat_server/objectbox.g.dart';
+import 'package:relic/relic.dart';
+
+import '../utils/extensions.dart';
+
+/// Public endpoint for cross-server identity verification
+/// Called by other servers to verify a user's public key
+/// 
+/// GET /.well-known/users/{username}
+/// Returns: { userId, publicKey, username, displayName, avatarUrl }
+Future<Response> publicUserHandler(Request req, String username) async {
+  final query = userBox.query(
+    User_.username.equals(username) & User_.isLocal.equals(true),
+  ).build();
+
+  final user = query.findFirst();
+  query.close();
+
+  if (user == null) {
+    return Response.notFound(
+      body: Body.fromString('User not found'),
+    );
+  }
+
+  return Response.ok(
+    body: {
+      'userId': user.userId,
+      'username': user.username,
+      'displayName': user.displayName,
+      'avatarUrl': user.avatarUrl,
+      'bio': user.bio,
+      'publicKey': user.publicKey,
+      'homeServer': Config.serverHost,
+      'createdAt': user.createdAt.toIso8601String(),
+    }.toBody,
+  );
+}
+
+/// Public endpoint to verify a user exists
+/// Lightweight check without full profile
+/// 
+/// GET /.well-known/verify/{userId}
+/// Returns: { exists, userId, publicKey }
+Future<Response> publicVerifyHandler(Request req, String userId) async {
+  final query = userBox.query(
+    User_.userId.equals(userId) & User_.isLocal.equals(true),
+  ).build();
+
+  final user = query.findFirst();
+  query.close();
+
+  if (user == null) {
+    return Response.ok(
+      body: {
+        'exists': false,
+        'userId': userId,
+      }.toBody,
+    );
+  }
+
+  return Response.ok(
+    body: {
+      'exists': true,
+      'userId': user.userId,
+      'publicKey': user.publicKey,
+      'username': user.username,
+    }.toBody,
+  );
+}
+
+/// Public server info
+/// 
+/// GET /.well-known/server
+/// Returns: { serverHost, serverName, version }
+Future<Response> publicServerInfoHandler(Request req) async {
+  return Response.ok(
+    body: {
+      'serverHost': Config.serverHost,
+      'serverName': Config.serverName,
+      'version': '0.1.0',
+      'features': ['identity', 'chat', 'sticker-packs'],
+    }.toBody,
+  );
+}