Make comment max length configurable (currently hardcoded to 1000)

Problem Comment length is hardcoded to 1000 characters in CommentValidation.maxLength (shared/validations.ts). The same constant is enforced in: - server model validation (server/models/Comment.ts) - comment editor UI (CommentForm maxLength) There is no env var or admin setting to change it. For self-hosted teams that use comments for longer review notes / feedback, 1000 characters is too short. Use case We run Outline 1.8.1 (outlinewiki/outline:1.8.1) and need longer document comments without forking/building a custom image. Proposal Allow self-hosters to raise the limit, for example: - an env var such as COMMENT_MAX_LENGTH with default 1000, or - another supported configuration approach the maintainers prefer The limit should stay consistent across server validation and the frontend editor. Notes - Comments are stored as JSONB, so raising the validation limit should not require a DB migration. - Related but different: #7103 (scroll …