Use siglum slug in manuscript URLs

Currently, URLs expose the internal ID number of the manuscript, which is meaningless. We already store the siglum slug, so this is a very easy change. We can redirect from the IDs to the slugs for backwards compatibility.

Start by locating where manuscript URLs are generated and where the stored siglum slug is exposed. Update the URL behavior to use the slug, preserve redirects from numeric IDs, and verify that both new slug URLs and legacy ID URLs resolve correctly.

Written by the indexing model from the issue text.