Add "Exact answer" search type to DB search

[Copilot]

Adds a new exactAnswer option to the search type dropdown, allowing users to find questions whose answer field starts with the query string. Unlike the existing answer type (substring match), this anchors the regex to the start of answer_sanitized/answers_sanitized, which aligns with quizbowl answer format where accept/prompt qualifiers follow the main answer in brackets (e.g., Albert Einstein [accept Einstein; ...]).

Changes

• database/qbreader/get-query.js

  • Added 'exactAnswer' to valid searchType values
  • getTossupQuery: matches answer_sanitized with ^${word} when searchType === 'exactAnswer'
  • getBonusQuery: matches answers_sanitized with ^${word} when searchType === 'exactAnswer'

• client/db/highlight-query.js: treats exactAnswer like answer for result highlighting

• client/db/index.jsx: adds <option value='exactAnswer'>Exact answer</option> to the search type <select>

The effective MongoDB query for tossups when using exactAnswer:

{ answer_sanitized: { $regex: `^${escapedQuery}`, $options: 'i' } }

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add "search for exact answer" mode to db search</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add "search for exact answer" mode to db search #447

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