feat: import a Calibre library

Reads metadata.db and copies the books into a library — from a zip uploaded on
the library settings page, or from a path with `litestar calibre-import`. The
source is never touched, and re-running only picks up what is new.

Also names the formats mimetypes does not know: a Calibre library is full of
MOBI and AZW3, and a null content type used to fail the book endpoint.

AGENTS.md

@@ -73,6 +73,9 @@ pytest tests/                                        # needs Docker (pytest-data
 ruff format src/
 alchemy --config chitai.database.config.config make-migrations
 alchemy --config chitai.database.config.config upgrade
+
+# Import a Calibre library. Copies files; --dry-run reports without writing.
+litestar --app-dir src/chitai/ calibre-import <path> --library <slug>
 ```
 
 Frontend (from `frontend/`):