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.

README.md

@@ -0,0 +1,44 @@
+# Mist
+
+A private Steam-clone for distributing games and updates to a small group of friends. Successor to the "Josh Steam" prototypes.
+
+## What it does
+
+- Hosts a catalog of games with version history
+- Generates **delta patches** between versions (hdiff for direct/consecutive jumps, librsync-style for indirect/arbitrary jumps) so friends download only what changed
+- Serves resumable downloads
+- Ships a desktop client (Tauri) so friends can browse, install, and update games like they would on Steam
+- Ships an admin web portal for managing the catalog
+- Ships a CLI (`mistpipe`) for uploading new game versions from your own machine
+
+## Repo layout
+
+```
+backend/        # FastAPI app + Celery worker (same image, two entrypoints)
+admin-web/      # SvelteKit admin portal
+client/         # Tauri desktop client (Rust shell + Svelte UI)
+mistpipe/       # Python CLI for admin uploads
+docs/           # ARCHITECTURE, DECISIONS, RUNBOOK
+.github/        # CI workflows
+docker-compose.yml      # production-ish stack
+docker-compose.dev.yml  # dev overlay (hot reload, bind mounts)
+```
+
+## Quickstart (development)
+
+```sh
+cp .env.example .env
+# edit .env, at minimum set passwords + JWT_SECRET
+
+docker compose -f docker-compose.yml -f docker-compose.dev.yml up --build
+```
+
+API at `http://localhost:8000`, admin web at `http://localhost:5173`, RabbitMQ UI at `http://localhost:15672`.
+
+For the client and CLI, see `client/README.md` and `mistpipe/README.md`.
+
+## Status
+
+Initial scaffold. The load-bearing `backend/src/mist/core/` modules (hdiff, librsync, chain-replay, manifest, compression, discord, steam, unrealpak) are ported from the original Josh Steam prototypes with bugs fixed and hardcoded paths replaced. Route handlers, UI screens, and CLI subcommands are skeletons — to be implemented in subsequent phases.
+
+See `docs/ARCHITECTURE.md` for the system design and `docs/DECISIONS.md` for the decision log.