Switch to cognito user/password authentication. Major code refactor.

app/models/auth.py

@@ -0,0 +1,20 @@
+from typing import List, Optional
+from pydantic import BaseModel, Field
+
+class SigninRequest(BaseModel):
+    """Request model for the signin endpoint."""
+    username: str = Field(..., description="The user's username")
+    password: str = Field(..., description="The user's password")
+
+class TokenResponse(BaseModel):
+    """Response model for successful authentication."""
+    access_token: str = Field(..., description="Access JWT token from Cognito")
+    id_token: str = Field(..., description="ID JWT token from Cognito")
+    refresh_token: Optional[str] = Field(
+        None, description="Refresh token from Cognito")
+    token_type: str = Field(..., description="Type of the token returned")
+
+class CognitoUser(BaseModel):
+    """Model representing the user returned from token verification."""
+    username: str
+    roles: List[str]