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/src-tauri/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "mist-client"
+version = "0.1.0"
+description = "Mist desktop client"
+authors = ["Josh"]
+edition = "2021"
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tauri = { version = "2", features = [] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[features]
+default = ["custom-protocol"]
+custom-protocol = ["tauri/custom-protocol"]

client/src-tauri/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    tauri_build::build()
+}

client/src-tauri/src/main.rs

@@ -0,0 +1,14 @@
+// Prevents an extra console window on Windows in release.
+#![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
+
+fn main() {
+    tauri::Builder::default()
+        // TODO: register #[tauri::command] handlers for:
+        //   - apply_hdiff_patch(old, patch, new) -> bool
+        //   - apply_librsync_patch(old, delta, new) -> bool
+        //   - generate_librsync_signature(file) -> Vec<u8>
+        //   - install_game(game_id, version) -> JobHandle
+        //   - launch_game(game_id) -> ()
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}

client/src-tauri/tauri.conf.json

@@ -0,0 +1,31 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "Mist",
+  "version": "0.1.0",
+  "identifier": "app.mist.client",
+  "build": {
+    "beforeDevCommand": "npm run dev",
+    "beforeBuildCommand": "npm run build",
+    "devUrl": "http://localhost:1420",
+    "frontendDist": "../dist"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "Mist",
+        "width": 1200,
+        "height": 800,
+        "minWidth": 900,
+        "minHeight": 600
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": []
+  }
+}