Add FastAPI backend and React frontend subprojects

Backend:
- FastAPI app with 17 REST endpoints covering dashboard, monitoring,
  media index, file browser, and jobs
- Reuses existing clients/domain/services unchanged
- pydantic-settings config, dependency injection, CORS setup
- Auto-generated OpenAPI docs at /docs

Frontend:
- Vite + React + TypeScript SPA
- @tanstack/react-query for data fetching with polling
- ag-grid-react for media table and file browser
- recharts for monitoring charts
- Tailwind CSS styling
- 4 pages: Dashboard, Monitoring, Media, File Browser
- Typed API client matching all backend endpoints

Also:
- docs/MIGRATION_PLAN.md with full architecture plan
- Updated .gitignore for both subprojects
- Streamlit app preserved for now (can coexist)

backend/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 clients.jellyfin import JellyfinClient
+from clients.ssh import RemoteSSHClient
+from 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"]