`map_folios.py` fails for IIIF Presentation 3 manifests

[kyrieb-ekat]

This would be the root cause of the wonkiness in the folios!

The folio mapping tool (cantusdata/views/map_folios.py) hardcodes IIIF
Presentation 2 manifest structure, causing it to fail for any manuscript
whose manifest uses the Presentation 3 format.

Root cause:

_extract_uris_from_manifest() accesses:

manifest_data["sequences"][0]["canvases"]

This key (sequences) does not exist in Presentation 3 manifests, which
use items instead. The canvas image structure also differs:

# Presentation 2
canvas["images"][0]["resource"]["service"]["@id"]

# Presentation 3
canvas["items"][0]["items"][0]["body"]["service"]["id"]

Additionally, the function fetches the manifest directly via urllib rather
than going through the manifest proxy, meaning none of the postprocessing
in cantusdata/helpers/postprocess_iiif.py is applied.

Steps to reproduce:

1. Add a manuscript with a IIIF Presentation 3 manifest URL via the admin
2. Import its chants
3. Navigate to /admin/map_folios/ and attempt to map the manuscript
4. Observe a KeyError: 'sequences'

Expected behaviour: The mapping tool correctly extracts image URIs from
both Presentation 2 and Presentation 3 manifests.

Suggested fix: Update _extract_uris_from_manifest() to detect the
manifest version via @context (as Diva's parser does) and branch
accordingly. Routing the fetch through the manifest proxy would also ensure
consistent postprocessing.