Org month-over-month contribution-change indicator

[peetzweg]

Part of #106 — the organization counterpart to the user month-over-month indicator (#41).

What to build

A CoinGecko-style "+X% vs last month" change indicator on org pages (and potentially org leaderboard rows) — the org-level twin of #41. "Your org did 50% more commits than last month."

Note this is a different data shape from the user indicator: user MoM reads existing per-user monthly_commits; org MoM needs the org monthly time-series from sub-issue C (orgs have no monthly storage today).

Acceptance criteria

• Org page shows month-over-month change (absolute and/or %) for commits, from stored org monthly rows
• Sensible handling of the in-progress current month (coordinate with Show the current in-progress month as a dashed 'trend' segment #35)
• Consistent visual language with the user indicator (Month-over-month contribution-change indicator (CoinGecko-style) #41)

Depends on

• Sub-issue C (org monthly time-series) — hard blocker
• Consistent with Month-over-month contribution-change indicator (CoinGecko-style) #41 (user MoM) and Show the current in-progress month as a dashed 'trend' segment #35 (in-progress month)