Release v0.1.0: tag, changelog and Docker images #135

Moved from #79's week-one list.

• Write the v0.1.0 changelog: everything since 7ce6795 (the original v0.0.0), mainly beehiiv switch-ready: everything needed before inviting beehiiv switchers #79's sub-issues (CSV import, billing and plans, abuse controls, test sends, auto-pause, signup forms, web archive, reputation, tracking domains and more).
• Tag v0.1.0 on main. The Docker workflow then publishes codelit/sendlit:0.1.0 / 0.1 and the matching sendlit-web images.
• Point QA and production deploys at 0.1.0 instead of the moving v0.0.0 tag.