Validates Postgres unquoted identifiers — the single source of truth for schema, prefix, and table-name checks across mailglass.

A valid identifier is a letter or underscore followed by any combination of letters, digits, and underscores, and is at most 63 bytes long (Postgres' NAMEDATALEN limit — longer names are silently truncated by Postgres, which can alias two distinct schemas onto one, so we reject them outright).

This grammar rejects anything that could be an injection vector — quotes, semicolons, whitespace, dashes. Callers with a legitimate need for a quoted identifier (mixed case, dashes) should pre-quote and adjust; mailglass does not surface such options.

Validation failures raise Mailglass.ConfigError with type: :invalid. The specific cause lives in context.key and context.reason — match on the struct, never on the message string.