docs: add mdBook docs-site, publish workflow, and release-notes tooling

Add a docs-site (mdBook) with a Gitea publish-docs workflow, a
release-notes generator script (release:notes) that turns a commit
range's TRACES into grouped notes, the background-audio feature spec,
and CLAUDE.md. Ignore docs-site build artifacts.

docs-site/SUMMARY.md

@@ -0,0 +1,39 @@
+# Summary
+
+[Introduction](README.md)
+
+# Requirements & Traceability
+
+- [Requirements Specification](requirements.md)
+- [Traceability Matrix](traceability.md)
+- [Traceability CI](traceability-ci.md)
+- [Traces Quick Reference](traces-quick-ref.md)
+
+# Architecture
+
+- [Overview](architecture/README.md)
+  - [Rust Backend](architecture/01-rust-backend.md)
+  - [Svelte Frontend](architecture/02-svelte-frontend.md)
+  - [Data Flow](architecture/03-data-flow.md)
+  - [Type Sync & Threading](architecture/04-type-sync-and-threading.md)
+  - [Platform Backends](architecture/05-platform-backends.md)
+  - [Downloads & Offline](architecture/06-downloads-and-offline.md)
+  - [Connectivity](architecture/07-connectivity.md)
+  - [Database Design](architecture/08-database-design.md)
+  - [Security](architecture/09-security.md)
+
+# UX & Specs
+
+- [UX Flows](ux-flows.md)
+- [Video Background Audio](specs/video-background-audio.md)
+
+# Build & Release
+
+- [Build & Release](build-release.md)
+- [Release Checklist](release-checklist.md)
+- [Docker](build/docker.md)
+- [Builder Image](build/build-builder-image.md)
+
+---
+
+[Rust API Reference (rustdoc)](api-redirect.md)