feat: add user model and registration endpoint

backend/app/schemas.py

@@ -0,0 +1,17 @@
+import uuid
+
+from pydantic import BaseModel, Field
+
+
+class RegisterRequest(BaseModel):
+    username: str = Field(min_length=3, max_length=32)
+    password: str = Field(min_length=8, max_length=128)
+    email: str | None = None
+
+
+class UserOut(BaseModel):
+    id: uuid.UUID
+    username: str
+
+    class Config:
+        from_attributes = True