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.

admin-web/Dockerfile

@@ -0,0 +1,14 @@
+# Build stage
+FROM node:22-alpine AS build
+WORKDIR /app
+COPY package.json ./
+RUN npm install --no-audit --no-fund
+COPY . .
+RUN npm run build
+
+# Serve stage
+FROM nginx:1.27-alpine
+COPY --from=build /app/build /usr/share/nginx/html
+EXPOSE 80
+# Default nginx config serves /usr/share/nginx/html; for SPA fallback, drop in a
+# minimal config here later if SvelteKit routes need the fallback to index.html.