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

.gitignore

@@ -0,0 +1,23 @@
+# Dart
+.dart_tool/
+.packages
+
+# ObjectBox generated
+*.g.dart
+objectbox-model.json
+
+# Build
+build/
+
+# Environment
+.env
+.env.local
+
+# IDE
+.idea/
+.vscode/
+*.iml
+
+# OS
+.DS_Store
+Thumbs.db