feat: raise WekanConnectionError on network failures and add request timeout

[bastianwenske]

Improves error handling in WekanClient.fetch_json:

• New WekanConnectionError (subclass of WekanAPIError) is raised when the server cannot be reached (requests ConnectionError/Timeout), instead of leaking raw requests exceptions.
• Requests now use a configurable timeout (WekanClient(..., timeout=30)), so calls can no longer hang forever.
• HTTP errors like a 502 from a reverse proxy already raise WekanAPIError with the status code since the exception refactoring; this is now covered by a unit test as well.

Fixes #16