Redact encryption master key from config validation error messages

[lgomezma]

Summary

The two key-length validation checks in lib/config.js interpolate the full encryptionMasterKey / encryptionMasterKeyBase64 value into the thrown Error. Since that key is the root secret for E2E encrypted channels and exceptions routinely propagate to logs, error-tracking services, CI output, and HTTP error responses, a length mismatch (e.g. a stray trailing newline, or passing a base64 key to the raw-bytes option) can persist the live key in those sinks.

This reports only the key length in the error, never the value. No other functional change.

Tests

Existing assertions match the byte-count text (… bytes long / decodes to N bytes), which is preserved — no test changes needed.