Initial Mist scaffold

Successor to the Josh Steam prototypes. Single-VM Docker Compose stack with
the load-bearing core/ logic ported from JoshSteam CDN with bug fixes.

Contents:
- backend/  FastAPI + Celery (same image, two entrypoints)
            core/  hdiff, librsync, chain_replay, manifest, compression,
                   discord, steam, unrealpak, paths
            api/   auth, catalog, admin, builds (skeletons) + downloads (real)
            worker/  Celery factory replacing the missing prototype Tasks/__init__.py
            db/    SQLAlchemy models + Alembic initial migration
- admin-web/  SvelteKit + Tailwind skeleton
- client/    Tauri 2 + Svelte skeleton (Mist placeholder UI)
- mistpipe/  click-based admin CLI with subcommand stubs
- docs/      ARCHITECTURE, DECISIONS (9 ADRs), RUNBOOK
- docker-compose.yml + dev overlay + .github/workflows

Bugs fixed during port:
- Routes/download.py:2 stray backslash on import line
- Utils/celery.py inspect.reserved() missing parens + double active() typo
- Hardcoded OneDrive/Desktop paths replaced with pydantic-settings config
- Discord webhook URL + RabbitMQ password moved to env vars
- Missing Tasks/__init__.py reconstructed as worker/__init__.py

Out of scope for this commit: route bodies, UI screens, mistpipe subcommand
bodies, real image builds.

client/README.md

@@ -0,0 +1,26 @@
+# Mist client
+
+The Tauri-based desktop client for Mist. Rust shell, Svelte UI inside.
+
+## Dev quickstart
+
+```sh
+pnpm install
+pnpm tauri dev
+```
+
+Tauri builds a native installer per platform via `pnpm tauri build`. Initial target is Windows (friends are on Windows); macOS/Linux straightforward to add.
+
+## What goes here
+
+- **`src/`** — Svelte UI (browse store, install, update, launch screens)
+- **`src-tauri/`** — Rust shell, native commands, file-system integration
+
+## Patch application
+
+The two patch tools (`hpatchz` for direct updates, `rdiff` for indirect) are external binaries. The plan is to either:
+
+1. Port `apply_hdiff_patch` / `apply_librsync_patch` to Rust using `tauri::command` so the UI invokes them natively, or
+2. Bundle a small Python sidecar binary built with PyInstaller that the Tauri shell spawns for each patch op (uses the same `mist.core.hdiff` / `mist.core.librsync` modules as the backend worker, except this side only *applies*, not *generates*).
+
+Decision deferred to the implementation phase.