Add module-level stats to the background stats worker

[adamdoupe]

Summary

The background stats worker currently only calculates stats at the dojo level (calculate_dojo_stats in dojo_plugin/worker/handlers/dojo_stats.py). We should extend it to also calculate and cache stats at the module level, then display them on the module page.

What exists today

• Dojo-level stats: calculated by the background worker, cached in Redis (stats:dojo:{reference_id}), displayed via stat_dashboard.html on dojo.html
• Module-level: module.solves() exists and can scope queries to a single module, but no stats are calculated or cached for modules
• Module page: shows per-challenge solve counts and a scoreboard, but no aggregate stats dashboard

What needs to be done

Backend

• Add calculate_module_stats(module) function (similar to calculate_dojo_stats but using module.solves())
• Add a module stats event handler in the worker (e.g., handle_module_stats_update)
• Cache module stats in Redis with key pattern stats:module:{dojo_reference_id}:{module_id}
• Fan out stats events to affected modules when solves happen (the listener already triggers on solve inserts)
• Add get_module_stats(module) retrieval function in dojo_plugin/utils/stats.py

Frontend

• Pass module stats from view_module() in dojo_plugin/pages/dojo.py to module.html
• Make stat_dashboard.html generic enough to render for either a dojo or a module (e.g., parameterize the header, conditionally show/hide cards like "Recent Awardees" which are dojo-specific)
• Include the stats dashboard on module.html

Stats to include per module

• Total Solves, Unique Hackers, Challenges count, Hacking Now (from container stats)
• Activity History chart (daily solves/users at snapshot intervals)
• Recent Solves
• Trend indicators (week-over-week changes)

Key files

• dojo_plugin/worker/handlers/dojo_stats.py - stats calculation and worker handler
• dojo_plugin/utils/stats.py - stats retrieval
• dojo_plugin/utils/listeners.py - event triggers
• dojo_plugin/utils/events.py - event publishing
• dojo_plugin/pages/dojo.py - view_module() controller
• dojo_theme/templates/module.html - module page template
• dojo_theme/templates/components/stat_dashboard.html - stats dashboard component