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/path_utils.py

@@ -0,0 +1,68 @@
+"""Path resolution utilities for Jellyfin → SSH path mapping."""
+
+from __future__ import annotations
+
+import posixpath
+
+
+def apply_remote_path_prefix(path: str, prefix: str) -> str:
+    """Apply an optional fallback prefix for Jellyfin->SSH path handoff."""
+    if not path:
+        return path
+    normalized_prefix = (prefix or "").strip()
+    if not normalized_prefix:
+        return path
+    normalized_prefix = normalized_prefix.rstrip("/")
+    if path == normalized_prefix or path.startswith(normalized_prefix + "/"):
+        return posixpath.normpath(path)
+    if path.startswith("/"):
+        return posixpath.normpath(normalized_prefix + path)
+    return posixpath.normpath(posixpath.join(normalized_prefix, path))
+
+
+def map_path_to_media_root(path: str, media_root: str) -> str:
+    """Map a Jellyfin path to the configured SSH media root when possible.
+
+    If the final segment of media_root (e.g. 'media') appears in the Jellyfin path,
+    the prefix up to that segment is replaced by media_root.
+    """
+    if not path:
+        return path
+    normalized_root = (media_root or "").strip()
+    if not normalized_root:
+        return path
+    normalized_root = posixpath.normpath(normalized_root)
+
+    raw_parts = [part for part in str(path).split("/") if part]
+    if not raw_parts:
+        return path
+
+    path_absolute = "/" + "/".join(raw_parts)
+    if path_absolute == normalized_root or path_absolute.startswith(normalized_root + "/"):
+        return path_absolute
+
+    root_anchor = posixpath.basename(normalized_root)
+    if not root_anchor:
+        return path
+
+    if root_anchor in raw_parts:
+        anchor_index = raw_parts.index(root_anchor)
+        remainder_parts = raw_parts[anchor_index + 1:]
+        return posixpath.join(normalized_root, *remainder_parts) if remainder_parts else normalized_root
+
+    return path
+
+
+def resolve_remote_media_path(path: str, media_root: str, fallback_prefix: str) -> str:
+    """Resolve Jellyfin paths to SSH-visible paths.
+
+    Strategy:
+    1. Prefer mapping to media_root when it can anchor on the root basename.
+    2. If no mapping happened, apply optional fallback prefix.
+    """
+    if not path:
+        return path
+    mapped = map_path_to_media_root(path, media_root)
+    if mapped and mapped != path:
+        return mapped
+    return apply_remote_path_prefix(mapped or path, fallback_prefix)