Add unique editorial content so the programmatic pages don't read as templated:
- gen_flavor.py seeds backend/cities_flavor.json with a short descriptive blurb per
city from Wikipedia's free REST summary API (no key), validating each match by
comparing article coordinates to the city's lat/lon so the wrong 'Springfield'
never attaches. Retries + modest concurrency; ~700/750 cities get a blurb, the
rest render without one. Text is CC BY-SA, attributed with a 'via Wikipedia' link.
- City pages show the blurb under the intro and a travel callout that deep-links to
the compare page with the city pre-loaded (/compare#loc=lat,lon) — the visitor
just adds their own city. Month pages get the same seasonal 'visiting in {month}?'
compare link. Both add unique per-page text and internal links.
- cities.py gains flavor(slug); tests cover the blurb + attribution + compare CTA.
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