…fix model types

- Added sig_payload query parameter for GET/DELETE requests (fixes 401 auth errors)
- Fixed List endpoint to pass query params in URL instead of body
- Fixed EventLog to parse {"logs": [...]} response instead of raw array
- Fixed LedgerItem.Amount type (server returns string, not float64)
- Fixed LedgerItem.CreatedAt type to string
- Fixed Event model ID to interface{} (server returns number)
- Fixed CredentialProfile.CardStorage type to interface{}
- Added Logo field to CreateTemplateParams
- Updated tests to match new response formats

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

The server returns RFC3339 timestamps which time.Time handles correctly.
Only Amount needed the type change (string vs float64).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>