Invalid regex


`type`	`https://docs.frem.sh/reference/api/errors/invalid-regex`
HTTP status	`400` Bad Request

What it means

The regex of a custom secret pattern is missing, longer than 300 characters, or not RE2 syntax. Backreferences and lookarounds are not supported.

Rewrite the pattern in RE2 syntax. detail carries the parser message.

All fremforge API errors are RFC 9457 problem+json:

{
  "type": "https://docs.frem.sh/reference/api/errors/invalid-regex",
  "title": "Bad Request",
  "status": 400,
  "detail": "..."
}

Branch on type, not on title or detail - those are human-readable and may be reworded.