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.

backend/pyproject.toml

@@ -0,0 +1,65 @@
+[build-system]
+requires = ["setuptools>=68", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "mist"
+version = "0.1.0"
+description = "Mist backend — FastAPI app + Celery worker for the Mist private game distribution platform"
+readme = "../README.md"
+requires-python = ">=3.12"
+license = { text = "Proprietary" }
+authors = [{ name = "Josh" }]
+
+dependencies = [
+    # Web framework
+    "fastapi>=0.115",
+    "uvicorn[standard]>=0.32",
+    "python-multipart>=0.0.12",     # multipart form parsing for builds upload
+    "pydantic>=2.9",
+    "pydantic-settings>=2.6",
+
+    # DB / migrations
+    "sqlalchemy>=2.0",
+    "alembic>=1.13",
+    "psycopg[binary]>=3.2",
+
+    # Auth
+    "argon2-cffi>=23.1",
+    "python-jose[cryptography]>=3.3",
+
+    # Background work
+    "celery>=5.4",
+    "redis>=5.1",
+    "kombu>=5.4",
+
+    # Content / compression
+    "zstandard>=0.23",
+    "py7zr>=0.22",
+
+    # Outbound HTTP
+    "httpx>=0.27",
+    "requests>=2.32",
+]
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=8.3",
+    "pytest-asyncio>=0.24",
+    "ruff>=0.7",
+    "mypy>=1.13",
+]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.ruff]
+line-length = 110
+target-version = "py312"
+
+[tool.ruff.lint]
+select = ["E", "F", "I", "B", "UP", "N"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]