refactor index schema and metadata format and directory tree

docs/development.md

@@ -40,6 +40,14 @@ uv run --extra gui python -m mediahive.winmain /path/to/media/folder
 
 This launches the same pywebview-based desktop flow used by the Windows build.
 
+## Migrate Existing Index Snapshots
+
+```bash
+uv run python scripts/indexmigr.py /path/to/media/root --write
+```
+
+This applies versioned snapshot migrations to `.mediahive/index.json` outside the main application. Use it before starting a newer build against an older index.
+
 ## Notes
 
 - The selected media folder is scanned continuously by the backend.

docs/multi-index-plan.md

@@ -35,9 +35,11 @@ Each active root gets an isolated `RootContext` managed by the `Supervisor`:
 
 ### Item IDs
 
-Every `Movie.id` and `Series.id` is namespaced with its `root_id`:
-- Format: `{root_id}:{content_hash}`
-- Old snapshots are auto-migrated on load: IDs lacking the prefix get it prepended.
+`root_id` is stored separately on each item.
+
+- `Movie.id` uses a slug built from the movie title and year, for example `spider-man-no-way-home-2021`.
+- `Series.id` uses a slug built from the series title, for example `lost`.
+- Legacy snapshot migrations are handled by `scripts/indexmigr.py`, not during app startup.
 
 ## API