chore: remove orphaned Event GraphQL fields (slug, matchingAlgorithm, emailTemplate)

[detail-app]

Removed three @Field-decorated properties from the Event ObjectType that had
no backing Prisma column, no @FieldResolver, no production callers, and no
references in EventEditInput or resolvers. Because they were declared as
non-nullable @field(() => String), any GraphQL query selecting them would
have errored at runtime — they were already broken.

• slug: never had an Event column in prisma/schema.prisma; the Survey column
  it relates to was added then dropped separately.
• matchingAlgorithm / emailTemplate: no Prisma column, no resolver, dead
  since the initial 2021 scaffold.

History

• Last materially changed January 2024 (05f7042)

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[augmentcode]

🤖 Augment PR Summary

Summary: Removes the orphaned slug, matchingAlgorithm, and emailTemplate fields from the GraphQL Event object type.
Technical note: These non-null fields had no Prisma columns or field resolvers, so selecting them could fail at runtime; the change aligns the GraphQL schema with the persisted Event model.

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[augmentcode]

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