fix: improve input validation, error logging, and env var handling

- Add validate_account_id() for get_account_holdings input validation
- Fix double logging bug in retry_on_auth_error decorator
- Remove emojis from log messages for cleaner log parsing
- Make PORT and LOG_LEVEL environment variables functional
- Delete redundant requirements.txt (pyproject.toml is authoritative)
- Clarify MONARCH_PORT is for Docker Compose only in .env.example

.env.example

@@ -13,5 +13,5 @@ MONARCH_MFA_SECRET=
 
 # Server Configuration
 PORT=8000
-MONARCH_PORT=8070
+MONARCH_PORT=8070  # Docker Compose host port mapping only
 LOG_LEVEL=INFO