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.

.env.example

@@ -0,0 +1,52 @@
+# Copy this file to `.env` and fill in real values. Never commit `.env`.
+
+# ---- Database ----
+POSTGRES_USER=mist
+POSTGRES_PASSWORD=changeme-postgres
+POSTGRES_DB=mist
+POSTGRES_HOST=postgres
+POSTGRES_PORT=5432
+DATABASE_URL=postgresql+psycopg://mist:changeme-postgres@postgres:5432/mist
+
+# ---- Redis ----
+REDIS_URL=redis://redis:6379/0
+
+# ---- RabbitMQ / Celery ----
+RABBITMQ_DEFAULT_USER=mist
+RABBITMQ_DEFAULT_PASS=changeme-rabbitmq
+CELERY_BROKER_URL=amqp://mist:changeme-rabbitmq@rabbitmq:5672//
+CELERY_RESULT_BACKEND=redis://redis:6379/1
+
+# ---- API ----
+API_HOST=0.0.0.0
+API_PORT=8000
+JWT_SECRET=changeme-generate-a-long-random-string
+JWT_ALG=HS256
+JWT_TTL_MINUTES=720
+CORS_ALLOWED_ORIGINS=https://store.example.com,https://admin.example.com
+
+# ---- Paths (inside the container) ----
+# NAS mount inside container; source of truth for game files
+GAMES_DIR=/mnt/nas/mist/games
+# Hot cache for prepared .tar.zst archives (Docker volume)
+CACHE_DIR=/mist/cache
+# Working / temp dir for in-flight delta-gen (Docker volume or tmpfs)
+TEMP_DIR=/mist/tmp
+
+# ---- Patch tool binaries (inside the container) ----
+# These are installed in the backend Dockerfile.
+HDIFFZ_PATH=/usr/local/bin/hdiffz
+HPATCHZ_PATH=/usr/local/bin/hpatchz
+RDIFF_PATH=/usr/local/bin/rdiff
+# Optional — only needed on the build host that ingests Unreal games
+UNREALPAK_PATH=
+
+# ---- External integrations ----
+DISCORD_WEBHOOK_URL=
+DISCORD_BOT_USERNAME=Mist
+DISCORD_BOT_AVATAR_URL=https://www.pcgamesn.com/wp-content/sites/pcgamesn/2018/10/gabe_newell_meme.jpg
+STEAM_API_KEY=
+
+# ---- Misc ----
+LOG_LEVEL=INFO
+ENVIRONMENT=development