Convert monthly reports to standard REST with shared filters

[maebeale]

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

• Making progress toward MonthlyReport being standalone instead of Report STI
• Standardize monthly reports to use the same REST conventions and filtered-index UX that workshop logs already use, so the two report types feel like siblings rather than parallel ad-hoc flows.
• Reduce duplication between MonthlyReport and WorkshopLog by extracting their shared model behavior into a Reportable concern.

How did you approach the change?

• Rewrote MonthlyReportsController against standard REST actions (index, show) and gave it a search/filter index that mirrors WorkshopLogsController: month/year, workshop, person, organization.
• Added the index/results/skeleton/search-boxes/quotable partials to match the workshop log UX.
• Extracted shared Report model behavior (associations, scopes, nested attrs, search, etc.) into app/models/concerns/reportable.rb, included by both MonthlyReport and the base Report.
• Removed monthly_reports/edit.html.erb since the new REST flow doesn't surface MR editing here.
• Updated routes: standard resources :monthly_reports, only: [ :index, :show ], plus a nested index under organizations. Dropped the legacy reports/monthly* routes.
• Preserved the previous controller as monthly_reports_controller_old.rb for reference during the cutover — will be removed once we're confident the new flow covers all legacy entry points.
• Beefed up the dummy dev seeds so the new index page has realistic data: ~89 monthly reports for Aisha (30 + a tapering tail), 5 for the admin, with form-field answers, occasional sectors, and occasional quotes.

UI Testing Checklist

• /monthly_reports index renders and lists reports
• Filters (month, year, workshop, person, organization) narrow results correctly
• Clearing filters returns the full list
• /monthly_reports/:id show renders for an existing report
• Organization show page links to MR index filtered by org
• Non-admin access matches policy (admin-or-auth)

Anything else to add?

• The _old controller file is intentionally committed for the cutover and will be deleted in a follow-up once the legacy entry points are confirmed dead.
• Follow-up PR will add the matching cross-model search additions (community_news, stories, event_registrations, story_ideas) and the organization "associated records" panel that links to all of these.