Activity log: writer, event subscribers and retention #2702

Problem

Who changed what, when and from which origin is required for governance and for safe agent writes; the log must be written for every change without each feature remembering to do so.

Proposal

Add cms_activity with the columns in spec section 4.9, an IActivityLog writer, event subscribers that log content, publishing, settings, site, role and key events with origin, client id and correlation id, a retention job (default 30 days, per site), and the query service the Management endpoint will use.

Acceptance

• Every event in the catalogue produces an activity row with origin and actor
• Retention removes rows past the configured age
• Queries filter by actor, origin, action, target and date range

Design: docs/planning/backend-technical-spec.md, section 4.9 (item B-31). Size: M.