rsvp_announcement: core fields and content type scaffold

Establish the announcement content type with all required fields: - Title (core) - HTML body with summary - field_media_image (entity reference to media) - field_external_link (optional link field) - field_expiration_date (date range — 180-day default is critical to town clerk auto-cleanup workflow) - field_show_read_more (boolean toggle) - field_show_post_date (boolean toggle) Form display: inline, logical grouping. View display: default + teaser. Gates: ADR-0022 must be Accepted.