Time-fenced "Top Tutorials" ranking (top in last 90/180/360 days) #1782

Follow-up from #1771 (deferred there).

Idea

Offer a completions-based "Top Tutorials" ranking with selectable time windows — top in the last 90 / 180 / 360 days — rather than (or alongside) the current homepage surface.

Context / why this is net-new

Investigation in #1771 established that the homepage "Featured missions" carousel ranks by KG PageRank (graph centrality, all-time, no time dimension) — see srv/lib/featured-topics-snapshot.js + srv/jobs/kg-pagerank-job.js. There is no completions/views-based popularity ranking anywhere on the homepage today (the personalized featuredOrder top-8 is hand-curated, not popularity). So a time-fenced "Top" list is a genuinely new feature, not a tweak.

Sketch

• Aggregate task completions from TaskRecords over a rolling window (90/180/360d), filtered to status='ACTIVE' or status is null (excludes INACTIVE/DELETED — already the convention).
• Materialize into a sidecar + nightly job (mirror the PageRank/featured-topics job pattern), or compute on read with caching.
• Surface as a homepage rail and/or a window selector.
• Decide relationship to the existing PageRank carousel (replace / complement / A-B).

Open questions

• Completions vs page views as the metric? (Completions are first-party + reliable; views need analytics.)
• Window default (180d?) and whether the selector is user-facing or admin config.

Refs #1771