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/src/mist/api/app.py

@@ -0,0 +1,60 @@
+"""FastAPI app factory.
+
+Run with: `uvicorn mist.api.app:app --host 0.0.0.0 --port 8000`
+"""
+
+from __future__ import annotations
+
+import logging
+from contextlib import asynccontextmanager
+from collections.abc import AsyncIterator
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from mist.api import admin, auth, builds, catalog, downloads
+from mist.config import settings
+from mist.core import paths as core_paths
+
+log = logging.getLogger(__name__)
+
+
+@asynccontextmanager
+async def lifespan(_: FastAPI) -> AsyncIterator[None]:
+    logging.basicConfig(level=settings.log_level)
+    log.info("Mist API starting; environment=%s", settings.environment)
+    core_paths.ensure_dirs()
+    yield
+    log.info("Mist API stopping")
+
+
+def create_app() -> FastAPI:
+    app = FastAPI(title="Mist API", version="0.1.0", lifespan=lifespan)
+
+    app.add_middleware(
+        CORSMiddleware,
+        allow_origins=settings.cors_origins or ["*"],
+        allow_credentials=True,
+        allow_methods=["*"],
+        allow_headers=["*"],
+    )
+
+    app.include_router(auth.router, prefix="/auth", tags=["auth"])
+    app.include_router(catalog.router, prefix="/catalog", tags=["catalog"])
+    app.include_router(admin.router, prefix="/admin", tags=["admin"])
+    app.include_router(builds.router, prefix="/builds", tags=["builds"])
+    app.include_router(downloads.router, prefix="/download", tags=["downloads"])
+
+    @app.get("/healthz", tags=["health"])
+    def healthz() -> dict[str, bool]:
+        return {"ok": True}
+
+    @app.get("/readyz", tags=["health"])
+    def readyz() -> dict[str, bool]:
+        # TODO: actually check db / redis / rabbitmq reachability
+        return {"ok": True}
+
+    return app
+
+
+app = create_app()