Restructure into backend/ and frontend/ subprojects

- backend/ uses proper Python src layout (src/media_library_viewer_api/)
  with pyproject.toml, hatchling build, and PYTHONPATH=src convention
- frontend/ is a Vite + React + TypeScript SPA
- archive/ preserves the original Streamlit prototype for reference
- Cleaned up root to only contain docs, license, and subproject dirs
- Updated README for the new dual-subproject architecture

backend/src/media_library_viewer_api/dependencies.py

@@ -0,0 +1,47 @@
+"""Dependency injection for FastAPI.
+
+Provides singleton-like access to SSH and Jellyfin clients via FastAPI's
+dependency system. Uses lru_cache so connections are reused across requests.
+"""
+
+from __future__ import annotations
+
+from functools import lru_cache
+
+from media_library_viewer_api.clients.jellyfin import JellyfinClient
+from media_library_viewer_api.clients.ssh import RemoteSSHClient
+from media_library_viewer_api.config import get_settings
+
+
+@lru_cache
+def get_jellyfin_client() -> JellyfinClient:
+    """Return a cached Jellyfin client."""
+    settings = get_settings()
+    return JellyfinClient(settings.jellyfin.url, settings.jellyfin.api_key)
+
+
+@lru_cache
+def get_ssh_client() -> RemoteSSHClient:
+    """Return a cached SSH client (connects on first use)."""
+    settings = get_settings()
+    client = RemoteSSHClient(
+        host=settings.ssh.host,
+        username=settings.ssh.username,
+        port=settings.ssh.port,
+        key_filename=settings.ssh.key_filename or None,
+        password=settings.ssh.password or None,
+    )
+    client.connect()
+    return client
+
+
+def get_user_id() -> str:
+    """Return the configured Jellyfin user ID, or discover the first available user."""
+    settings = get_settings()
+    if settings.jellyfin.user_id:
+        return settings.jellyfin.user_id
+    client = get_jellyfin_client()
+    users = client.users()
+    if not users:
+        raise RuntimeError("No Jellyfin users found and JELLYFIN_USER_ID not set")
+    return users[0]["Id"]