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/config.py

@@ -0,0 +1,63 @@
+"""Backend configuration using pydantic-settings.
+
+Reads from environment variables and .env file automatically.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from pydantic_settings import BaseSettings
+
+
+class JellyfinSettings(BaseSettings):
+    url: str = ""
+    api_key: str = ""
+    user_id: str = ""
+
+    model_config = {"env_prefix": "JELLYFIN_"}
+
+
+class SSHSettings(BaseSettings):
+    host: str = ""
+    username: str = ""
+    port: int = 22
+    key_filename: str = str(Path.home() / ".ssh" / "id_rsa")
+    password: str = ""
+    media_root: str = ""
+    path_prefix: str = ""
+
+    model_config = {"env_prefix": "SSH_"}
+
+
+class RemoteSettings(BaseSettings):
+    """Extra remote settings that don't fit the SSH_ prefix."""
+
+    media_root: str = ""
+    path_prefix: str = ""
+
+    model_config = {"env_prefix": "REMOTE_"}
+
+
+class Settings(BaseSettings):
+    """Top-level application settings."""
+
+    jellyfin: JellyfinSettings = JellyfinSettings()
+    ssh: SSHSettings = SSHSettings()
+    remote: RemoteSettings = RemoteSettings()
+
+    # Derived convenience properties
+    @property
+    def media_root(self) -> str:
+        return self.remote.media_root or self.ssh.media_root or ""
+
+    @property
+    def path_prefix(self) -> str:
+        return self.remote.path_prefix or self.ssh.path_prefix or ""
+
+    model_config = {"env_file": ".env", "env_file_encoding": "utf-8", "extra": "ignore"}
+
+
+def get_settings() -> Settings:
+    """Create a Settings instance (reads env/.env on each call)."""
+    return Settings()