fix: correct tojson|safe usage in templates and update validator

- Remove |safe from |tojson in HTML attributes (x-data) - quotes must
  become " for browsers to parse correctly
- Update LANG-002 and LANG-003 architecture rules to document correct
  |tojson usage patterns:
  - HTML attributes: |tojson (no |safe)
  - Script blocks: |tojson|safe
- Fix validator to warn when |tojson|safe is used in x-data (breaks
  HTML attribute parsing)
- Improve code quality across services, APIs, and tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/letzshop/client_service.py

@@ -30,20 +30,14 @@ class LetzshopClientError(Exception):
 class LetzshopAuthError(LetzshopClientError):
     """Raised when authentication fails."""
 
-    pass
-
 
 class LetzshopAPIError(LetzshopClientError):
     """Raised when the API returns an error response."""
 
-    pass
-
 
 class LetzshopConnectionError(LetzshopClientError):
     """Raised when connection to the API fails."""
 
-    pass
-
 
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