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.

docker-compose.dev.yml

@@ -0,0 +1,54 @@
+# Mist — development overlay.
+# Use: `docker compose -f docker-compose.yml -f docker-compose.dev.yml up --build`
+#
+# Differences from prod:
+#   - Builds images locally instead of pulling from GHCR
+#   - Bind-mounts backend src for hot reload (uvicorn --reload)
+#   - Exposes infra ports so you can connect from your host
+
+services:
+  postgres:
+    ports:
+      - "5432:5432"
+
+  redis:
+    ports:
+      - "6379:6379"
+
+  rabbitmq:
+    ports:
+      - "5672:5672"
+      - "15672:15672"   # management UI
+
+  api:
+    image: mist-backend:dev
+    build:
+      context: ./backend
+    command:
+      [
+        "uvicorn",
+        "mist.api.app:app",
+        "--host",
+        "0.0.0.0",
+        "--port",
+        "8000",
+        "--reload",
+      ]
+    volumes:
+      - ./backend/src:/app/src
+
+  worker:
+    image: mist-backend:dev
+    build:
+      context: ./backend
+    # Watch the source tree and restart on changes (celery doesn't auto-reload).
+    # For dev, just restart the container manually after big changes.
+    volumes:
+      - ./backend/src:/app/src
+
+  admin-web:
+    image: mist-admin-web:dev
+    build:
+      context: ./admin-web
+    volumes:
+      - ./admin-web/src:/app/src