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.

.gitignore

@@ -0,0 +1,75 @@
+# Secrets
+.env
+.env.local
+.env.*.local
+
+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.so
+.Python
+*.egg
+*.egg-info/
+.eggs/
+.venv/
+venv/
+env/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+.tox/
+.coverage
+.coverage.*
+htmlcov/
+dist/
+build/
+*.whl
+
+# Node
+node_modules/
+.svelte-kit/
+.vite/
+.turbo/
+*.tsbuildinfo
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+pnpm-debug.log*
+.pnpm-store/
+
+# Rust / Tauri
+target/
+Cargo.lock
+client/src-tauri/gen/
+
+# Editor / OS
+.DS_Store
+Thumbs.db
+.idea/
+.vscode/
+*.swp
+*~
+
+# Build artifacts
+*.tar.zst
+*.zst
+*.7z
+*.tmp
+*.patch
+*.sig
+*.dlt
+
+# Local data / cache (should never be in repo)
+data/
+cache/
+tmp/
+games/
+mist-data/
+
+# Logs
+*.log
+logs/
+
+# Docker
+docker-compose.override.yml