Chapter health card on the admin chapter page

[mroderick]

I'm trying to think of ways to make it easier for admins and organisers alike to have better insights into the health of chapters. I think that workshop frequency and number of active organisers are some good indicators, so I've tried to capture this in a health card that is displayed at the top of the chapter admin page.

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Adds a Chapter Health card to the admin chapter page so admins and chapter organisers can see a chapter's health at a glance, without visiting the separate Chapter Status page.

• Status badge (active / dormant / inactive) using the same classification rules as the Chapter Status page
• Previous workshop (days ago) and next workshop (days away), with exact dates on hover
• Median workshop cadence over the past 180 days
• Eligible members (subscribed, not banned, TOC accepted) and organiser count
• Workshop and events timeline: SVG, 180 days past / 90 days future, today marked; workshops as blue circles, single-chapter events as amber squares; same-day events stack with an xN count. Pure SVG plus native tooltips, no JavaScript

Design and review notes

• The card is visible to admins and the chapter's organisers. Chapter show was already organiser-accessible, and the data shown is the chapter's own, so no new access is granted.
• Single-chapter events only: multi-chapter events list 33-44 chapters each, so they carry no per-chapter signal (production data: 13 events in the last 180 days cover 36 chapters). New Event.single_chapter scope.
• Classification logic mirrors Admin::ChaptersController#status inline rules; the two implementations coexist by design (status page stays unchanged).
• Timeline date basis is midnight-aligned so tick labels and markers cannot drift at DST boundaries.
• Reviews applied: layered-rails (scope extraction, view arithmetic moved to the value object, spec coverage restored), ponytail-review (dead code, inlined single-caller module), ce-code-review (test gap on the new scope).
• Residual cosmetic edge: more than three same-day events stack only three markers; the xN label still shows the true count.