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/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "mist-client",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "vite build",
+    "preview": "vite preview",
+    "tauri": "tauri",
+    "check": "svelte-check --tsconfig ./tsconfig.json"
+  },
+  "dependencies": {
+    "@tauri-apps/api": "^2.1.0"
+  },
+  "devDependencies": {
+    "@sveltejs/vite-plugin-svelte": "^4.0.0",
+    "@tauri-apps/cli": "^2.1.0",
+    "svelte": "^5.0.0",
+    "svelte-check": "^4.0.0",
+    "tslib": "^2.7.0",
+    "typescript": "^5.6.0",
+    "vite": "^5.4.0"
+  }
+}