feat: add automatic re-authentication with MFA support

Implement automatic token refresh using stored credentials and TOTP MFA secret. When an API call fails with a 401/unauthorized error, the system now transparently re-authenticates using MONARCH_EMAIL, MONARCH_PASSWORD, and MONARCH_MFA_SECRET, then retries the original request.

Changes:
- Add refresh_authentication() function in auth.py for credential-based login
- Create @retry_on_auth_error decorator to handle and retry failed auth calls
- Apply decorator to all MCP tools (get_accounts, get_transactions, etc.)
- Add MONARCH_MFA_SECRET to .env.example with documentation
- Update login_setup.py to instruct users about required env vars
- Replace PROBLEM.md with PLAN.md documenting the implementation

.env.example

@@ -2,10 +2,15 @@
 # You can use MONARCH_TOKEN (recommended) OR Email/Password
 MONARCH_TOKEN=
 
-# Fallback credentials
+# Credentials for automatic re-authentication (required for token refresh)
 MONARCH_EMAIL=
 MONARCH_PASSWORD=
 
+# MFA Secret for TOTP-based re-authentication
+# This is the secret you saved when you first set up 2FA for Monarch Money
+# Required if you have MFA enabled on your Monarch account
+MONARCH_MFA_SECRET=
+
 # Server Configuration
 PORT=8000
 MONARCH_PORT=8070