Dynamic features driven by per-post read counts

## Goal

Use per-post read counts (sourced from #141) to add lightweight dynamic surfaces that help readers find what's resonating and give me a feedback loop on what's landing.

Blocked by #141 — needs an analytics source first.

## Candidate features

- **Popular posts widget** on the homepage / sidebar — top N by reads over a rolling window (e.g. last 90 days).
- **Per-post read counter** displayed near the title or in the meta block.
- **\"Trending now\"** — top N over a short window (e.g. last 7 days), only shown when a post crosses some threshold to avoid noise.

Pick one to ship first; the others can follow as separate PRs.

## Implementation sketch

Static site, so the data has to be baked at build time:

- Build step queries the Cloudflare GraphQL Analytics API for page-view counts per path.
- Result written to a JSON file under \`src/data/\` (or pulled directly via Astro content layer loader).
- Components read the JSON.
- Build runs daily via a scheduled workflow, or on every \`main\` push (latter is cheaper but staler between posts).

Token for the GraphQL API stored as a repo secret.

## Out of scope

- Real-time counts (would need client-side fetch; not worth the complexity for a personal blog).
- Scroll-depth-weighted rankings — see follow-up issue.

## Open questions

- Cadence: rebuild on push only, or also on a daily schedule?
- Recency window for \"popular\" — 90 days, all-time, or both?
- Display format for counts (exact, rounded, log-bucketed)?