Add asciifolding filter to keyword_normalizer for consistent accent-insensitive search

[Copilot]

Addresses review feedback on #276 to ensure accent-insensitive search behavior applies consistently across both analyzed text fields and keyword fields.

Changes

• Added asciifolding filter to keyword_normalizer in ElasticSearch configuration

Previously, asciifolding was only applied to my_analyzer (used for text fields like title/description). Keyword fields (tags, autoTags, textTags) used keyword_normalizer without asciifolding, creating inconsistent search behavior where searching for "Vezelay" would match text content containing "Vézelay" but not tags containing "Vézelay".

normalizer:
  keyword_normalizer:
    type: custom
    filter: [lowercase, asciifolding]  # added asciifolding

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