Add year filter for frequency lists

[Copilot]

Frequency lists had no way to filter by year range, making it impossible to scope results to a specific era of questions.

Changes

• database/qbreader/get-frequency-list.js: Accept minYear/maxYear and apply as set.year range filter in the MongoDB match document
• routes/api/frequency-list.js: Validate and pass through minYear/maxYear query params; treated as optional (absent = no bound)
• client/db/frequency-list/subcategory.html: Add year range slider UI (same component as play pages)
• client/db/frequency-list/subcategory.jsx: Wire up slider events, track minYear/maxYear state, include in API request via existing filterParams utility
• client/tools/api-docs/frequency-list.html: Document new minYear / maxYear parameters

API

The new parameters follow the same conventions as query and random-tossup/random-bonus:

GET /api/frequency-list?subcategory=Biology&minYear=2015&maxYear=2023

Both are optional integers (inclusive bounds). Omitting either applies no bound on that side.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add year filter for frequency lists</issue_title>
<issue_description></issue_description>

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

• Fixes Add year filter for frequency lists #440

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