feat: add Internet Culturale (ICCU) provider

[nikazzio]

Adds full support for the ICCU aggregator (internetculturale.it) which covers
Biblioteca Medicea Laurenziana, Biblioteca Nazionale Marciana, BNCF, BNCR and
~50+ Italian institutions via the MAG/XML API.

Changes:

• resolvers/mag_parser.py: MAG XML → IIIF v2 manifest converter
  • parse_mag_xml(): parse bibinfo, pages, build standard IIIF v2 manifest
  • IccuMetadata: structured metadata (library, city, sbn_code, shelfmark, oai_id, teca)
  • fetch_and_convert(): download + convert magparser endpoint
  • is_iccu_magparser_url(): intercept hook for downloader pipeline
  • Uses defusedxml for safe XML parsing
• resolvers/internetculturale.py: resolver class
  • can_resolve(): matches internetculturale.it URLs and known OAI prefixes
  • get_manifest_url(): extracts OAI ID + teca, returns magparser URL
  • OAI prefix → teca lookup table (Marciana, BML, MagTeca-ICCU)
• resolvers/search/internetculturale.py: HTML scraper for IC manuscript search
  • Targets /it/16/search?instance=magindice&channel__typeTipo=Manoscritto
  • Extracts OAI ID, teca, title, author, date, library, thumbnail per result
• providers.py: register Internet Culturale provider (sort_order=5, first in UI)
• logic/downloader.py: intercept magparser URLs before get_json() call
• resolvers/discovery.py + search/init.py: wire search_internetculturale
• tests/test_providers.py: update sort order assertion

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com