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

@@ -48,4 +48,5 @@ prompt
 
 server.log
 # Skills directory
-/skills/
+/skills/download/
+*.g.dart