SEO: per-city Wikipedia blurbs + travel/compare CTA on city pages (#98)

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.

static/style.css

@@ -1477,6 +1477,12 @@ table.normals-table tbody tr:hover { background: var(--surface-2); }
 .climate-records ul { margin: 8px 0; padding-left: 20px; }
 .climate-records li { margin: 4px 0; }
 .climate-foot { font-size: 14px; margin-top: 28px; border-top: 1px solid var(--border); padding-top: 16px; }
+.city-blurb { font-size: 15px; margin: 4px 0 14px; }
+.blurb-src { color: var(--muted); text-decoration: none; font-size: 13px; white-space: nowrap; }
+.blurb-src:hover { color: var(--accent); }
+.city-travel { background: var(--surface); border: 1px solid var(--border); border-radius: 12px; padding: 14px 18px; margin: 28px 0; }
+.city-travel h2 { margin-top: 4px; }
+.travel-note { font-size: 15px; }
 
 /* Hub / directory of cities */
 .hub-country { margin: 22px 0; }