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.yml

@@ -0,0 +1,97 @@
+# Mist — production-ish Docker Compose stack.
+# Brings up: postgres, redis, rabbitmq, api, worker, admin-web.
+# All env config comes from .env (copy from .env.example).
+
+services:
+  postgres:
+    image: postgres:16-alpine
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+      POSTGRES_DB: ${POSTGRES_DB}
+    volumes:
+      - postgres-vol:/var/lib/postgresql/data
+    networks:
+      - mist-net
+
+  redis:
+    image: redis:7-alpine
+    restart: unless-stopped
+    volumes:
+      - redis-vol:/data
+    networks:
+      - mist-net
+
+  rabbitmq:
+    image: rabbitmq:3.13-management-alpine
+    restart: unless-stopped
+    environment:
+      RABBITMQ_DEFAULT_USER: ${RABBITMQ_DEFAULT_USER}
+      RABBITMQ_DEFAULT_PASS: ${RABBITMQ_DEFAULT_PASS}
+    volumes:
+      - rabbitmq-vol:/var/lib/rabbitmq
+    networks:
+      - mist-net
+
+  api:
+    image: ghcr.io/REPLACE_ME/mist-backend:latest
+    build:
+      context: ./backend
+    restart: unless-stopped
+    depends_on:
+      - postgres
+      - redis
+      - rabbitmq
+    env_file:
+      - .env
+    volumes:
+      - cache-vol:/mist/cache
+      - tmp-vol:/mist/tmp
+      # TODO: bind-mount NAS NFS export here. Configure host-side NFS mount,
+      # then uncomment the line below and point it at the host mount point.
+      # - /mnt/nas:/mnt/nas:rw
+    ports:
+      - "8000:8000"
+    networks:
+      - mist-net
+
+  worker:
+    image: ghcr.io/REPLACE_ME/mist-backend:latest
+    build:
+      context: ./backend
+    restart: unless-stopped
+    command: ["celery", "-A", "mist.worker", "worker", "--loglevel=INFO"]
+    depends_on:
+      - postgres
+      - redis
+      - rabbitmq
+    env_file:
+      - .env
+    volumes:
+      - cache-vol:/mist/cache
+      - tmp-vol:/mist/tmp
+      # - /mnt/nas:/mnt/nas:rw
+    networks:
+      - mist-net
+
+  admin-web:
+    image: ghcr.io/REPLACE_ME/mist-admin-web:latest
+    build:
+      context: ./admin-web
+    restart: unless-stopped
+    ports:
+      - "5173:80"
+    networks:
+      - mist-net
+
+volumes:
+  postgres-vol:
+  redis-vol:
+  rabbitmq-vol:
+  cache-vol:
+  tmp-vol:
+
+networks:
+  mist-net:
+    driver: bridge