Feature Idea: Newsletter/Publication as an optional UI concept grouping lists, templates, and campaigns

Is your feature request related to a problem? Please describe. Listmonk’s data model separates lists, campaigns, and templates into independent entities with no persistent relationship between them. For single-newsletter use cases this works fine, but for anyone running multiple independent newsletters on one instance — each with its own sender identity, subscriber base, branding, and opt-in flow — the lack of a grouping concept creates real operational risk and friction. Concretely: nothing prevents a campaign intended for Newsletter A from being sent with Newsletter B’s From address, Newsletter B’s template, or to Newsletter B’s subscribers. Listmonk does not validate or warn. At two newsletters this is a procedural check; at ten it stops being survivable — as #3179 describes in detail, with a patch already in production. This is the same structural gap behind several open requests: • #3179 — per-list default From address, because campaigns don’t know which sender they belon…