Add Italian comics support

[bpepple]

Description

Adds initial support for Italian comics, as discussed in #595:

Schema

• Publisher.alt_names — array field for alternative publisher names (mirrors Series.alt_names), with a matching GIN trigram index
• Series.language — ISO 639-1 language code, defaults to "en"
• Issue.price / Variant.price — max_digits widened from 5 to 8 to support larger cover price amounts
• CURRENCIES (settings) — adds EUR and ITL alongside USD/GBP; this is a project-wide djmoney setting, so CollectionItem.purchase_price_currency and WishListItem.max_price_currency picked up matching migrations too
• COUNTRIES (settings) — new setting (US, GB, IT) backing PublisherForm's country validation, replacing a hardcoded set

Admin / forms / filters

• alt_names and language wired into PublisherAdmin/SeriesAdmin and their forms
• SeriesForm.clean_language and PublisherForm.clean_country restrict the web UI to the currently-supported set (en/it, US/GB/IT) — the API itself is intentionally left unrestricted, since existing data already includes publishers outside that set (e.g. CA, FR)
• New PublisherFilter (API) with alt_names and a q quick-search filter across name/alt_names, mirroring SeriesFilter
• Web publisher search box extended to also match alt_names
• language filter added to SeriesFilter/SeriesViewFilter, including the web UI's advanced-filters dropdown and active-filter chips

Templates

• Publisher detail page shows alternative names; Series detail page shows language

API

• alt_names exposed on PublisherSerializer; language exposed on SeriesSerializer and the nested series object in IssueSerializer (so MetronInfo.xml writers can read it from the issue detail endpoint)
• api/README.md updated throughout (new fields/filters, corrected a stale claim that the API restricted country to US/GB, documented previously-undocumented Variant price/price_currency fields, new "Version 1.10" changelog entry)

i18n

• locale/it/LC_MESSAGES/django.po updated with the new/changed strings this branch introduces

Checklist

• Tests pass locally (pytest)
• Linted (ruff check . --fix && ruff format .)
• New/changed user-facing strings in templates, forms, and views are wrapped for translation, and locale/it/LC_MESSAGES/django.po was updated
• Migrations included