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/core/compression.py

@@ -0,0 +1,56 @@
+"""zstd streaming compression + 7z extraction.
+
+Ported from: JoshSteam CDN/Utils/compression.py
+No behavior changes; just type hints and path-objects.
+"""
+
+from __future__ import annotations
+
+import os
+import tarfile
+from pathlib import Path
+from typing import BinaryIO
+
+import py7zr
+import zstandard as zstd
+
+
+def generate_tar_zstd_stream(depot_path: Path | str, output_stream: BinaryIO) -> None:
+    """Stream tarball of `depot_path` contents into an output binary stream."""
+    depot_path = Path(depot_path)
+    with tarfile.open(fileobj=output_stream, mode="w|") as tar:
+        for root, _, files in os.walk(depot_path):
+            for file in files:
+                file_path = Path(root) / file
+                relative_path = file_path.relative_to(depot_path)
+                tar.add(file_path, arcname=str(relative_path))
+
+
+def compress_and_save_zstd(
+    depot_path: Path | str, temp_zstd_path: Path | str, final_zstd_path: Path | str
+) -> None:
+    """Tar+zstd `depot_path` to a temp file, then atomically rename to final.
+
+    No intermediate uncompressed .tar is created on disk.
+    """
+    temp_zstd_path = Path(temp_zstd_path)
+    final_zstd_path = Path(final_zstd_path)
+    temp_zstd_path.parent.mkdir(parents=True, exist_ok=True)
+
+    with open(temp_zstd_path, "wb") as out_file:
+        cctx = zstd.ZstdCompressor(level=3)
+        with cctx.stream_writer(out_file) as compressor:
+            generate_tar_zstd_stream(depot_path, compressor)
+
+    os.replace(temp_zstd_path, final_zstd_path)
+
+
+def extract_7z(archive_path: Path | str, destination_path: Path | str) -> None:
+    """Extract a .7z archive into `destination_path`."""
+    archive_path = Path(archive_path)
+    destination_path = Path(destination_path)
+    if not archive_path.exists():
+        raise FileNotFoundError(f"The archive {archive_path} does not exist.")
+    destination_path.mkdir(parents=True, exist_ok=True)
+    with py7zr.SevenZipFile(archive_path, mode="r") as archive:
+        archive.extractall(path=destination_path)