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/tests/conftest.py

@@ -0,0 +1 @@
+"""Test configuration. Empty for now — present so pytest discovers the tests/ dir."""

backend/tests/test_smoke.py

@@ -0,0 +1,26 @@
+"""Smoke test: every module imports cleanly.
+
+If a dep is missing, a syntax error slipped through, or a circular import got
+introduced, this test fails. Cheap insurance.
+"""
+
+
+def test_core_modules_import() -> None:
+    from mist.core import (  # noqa: F401
+        chain_replay,
+        compression,
+        discord,
+        hdiff,
+        librsync,
+        manifest,
+        paths,
+        steam,
+        unrealpak,
+    )
+
+
+def test_config_loads() -> None:
+    from mist.config import settings
+
+    assert settings.environment in {"development", "staging", "production"} or settings.environment
+    # don't assert specific values; just that it loaded