Add workshops dropdown filter to workshop_logs index

[Copilot]

What is the goal of this PR and why is this important?

Enable filtering workshop logs by workshop without degrading page performance. The original attempt used Workshop.joins(:workshop_logs) which joined all workshop logs regardless of user permissions or active filters, causing slow queries.

How did you approach the change?

Changed query strategy from JOIN to subquery:

# Before: joins all workshop_logs in database
Workshop.joins(:workshop_logs).order(:title)

# After: selects only from already-filtered logs
Workshop.where(id: @workshop_logs_unpaginated.select(:workshop_id).distinct)
        .order(:title)

This approach:

• Leverages existing index on reports.workshop_id
• Respects permission filters already applied to @workshop_logs_unpaginated
• Scales with filtered result size, not total database size
• Works seamlessly with existing filters (month, year, facilitator, organization)

View changes:

• Added workshop dropdown between Year and Facilitator filters
• Auto-submits on selection change
• Follows existing filter pattern and styling

Test coverage:

• Index page loads successfully
• Filtering by workshop_id parameter works correctly
• Workshop dropdown populates from visible logs only

Anything else to add?

The WorkshopLog model already had the workshop_id scope and search method support, so only the controller query and view needed changes. No migrations or new dependencies required.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add workshops dropdown/filter on /workshop_logs</issue_title>
<issue_description>I had it rigged up but the db must be missing an index or something bc it makes the page perform pretty slowly</issue_description>

<agent_instructions>find a lightweight way to add a workshops dropdown to workshop_logs without crushing the performance of the workshop_logs index.</agent_instructions>

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

@maebeale @copilot find a lightweight way to add a workshops dropdown to workshop_logs without crushing the performance of the workshop_logs index.

• Fixes Add workshops dropdown/filter on /workshop_logs #209

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