Add CHANGELOG.md, serve it in-app at /docs/changelog (closes theta42/theta-env#43)

GitHub Releases already carried real changelog notes per tag, but
those require internet access to view -- exactly what the /docs
route exists to avoid. CHANGELOG.md is a committed, Keep-a-Changelog
style file (backfilled from the v1.1.0/v1.1.1/v1.1.2 release notes),
linked from README and served at /docs/changelog alongside the rest
of the project's docs.

CHANGELOG.md

@@ -0,0 +1,35 @@
+# Changelog
+
+All notable changes to this project are documented here. Format loosely
+follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); versions
+correspond to git tags (`vX.Y.Z`) and `nodejs/package.json`'s `version`.
+
+## [Unreleased]
+
+## [1.1.2] - 2026-07-16
+
+### Fixed
+- **Air-gap**: `DynamicRecord.refreshAll()` called the public-IP resolvers (`api.ipify.org`, `icanhazip.com`, `ifconfig.me`) every 4h on a timer regardless of whether any dynamic DNS records were configured — the one background network call in the repo not actually gated by feature use. Now skips the lookup entirely when there's nothing to refresh.
+- Removed the stray, unauthenticated `GET /test` page (a leftover jq-repeat demo) that loaded jQuery + Mustache from external CDNs.
+- Removed a dead IE<9-only `html5shim` script tag pointing at a domain that no longer resolves.
+
+### Added
+- **In-app documentation**: `GET /docs` and `GET /docs/:slug` render this project's own README, DEPLOYMENT, `api.md`, and `docs/*.md` server-side — readable from the running app with no dependency on GitHub Pages, which requires internet access to view. Public, no auth, rate-limited.
+
+## [1.1.1] - 2026-07-16
+
+### Fixed
+- **DuckDNS provider**: adding a DuckDNS provider no longer pushes this host's public IP to the domain's live A/AAAA record as a side effect of token validation. Validation now writes a fixed marker to the TXT record instead, leaving routing untouched. ([#142](https://github.com/theta42/proxy/pull/142))
+
+## [1.1.0] - 2026-07-16
+
+First tagged release. Establishes the `vX.Y.Z` tag convention that the in-app update-check banner polls against going forward.
+
+### Added
+- Standalone backup script (`ops/backup.sh`) for deployments not using theta-env's orchestrator — snapshots Redis and `./config`, with retention.
+- Admin-only in-app banner that checks GitHub releases every 24h and surfaces available updates.
+
+[Unreleased]: https://github.com/theta42/proxy/compare/v1.1.2...HEAD
+[1.1.2]: https://github.com/theta42/proxy/compare/v1.1.1...v1.1.2
+[1.1.1]: https://github.com/theta42/proxy/compare/v1.1.0...v1.1.1
+[1.1.0]: https://github.com/theta42/proxy/releases/tag/v1.1.0