feat: add per-city landing pages generated from the places dataset

[aryansk]

Problem

Closes #127. We hold places for hundreds of cities but only one indexable page; someone searching "SAT exam centre in Pune" finds nothing.

What this adds

• Statically generated page per city at /city/[slug], one for every distinct city value in the dataset (217 pages), using generateStaticParams — ● (SSG) in the build output, no per-request rendering.
• Slug derived from the existing city field — no second city list or slug mapping file; the dataset stays the single source of truth.
• Grouped by category in canonical PLACE_TYPES order, with place counts.
• City name in <title>, <h1>, and meta description (humanizeCity for display).
• Map deep links: each place links to /map?place=<id> (the place param the map already reads) plus its Google Maps link.
• Thin cities (≤ 3 places) get a "just getting started" card with a prompt to contribute more places.
• Collision guard: two distinct city values that slugify identically fail the build loudly instead of silently merging (the dataset currently has zero collisions).
• City routes added to the sitemap.

Notable detail

The dataset includes one non-ASCII city (厦门) and one with a space (new delhi). The slugifier preserves Unicode letters (so 厦门 never collapses to an empty slug) and normalizes the space case to new_delhi; the page decodes the route param before matching.

Verification

• npm run validate passes (6 files clean).
• 46 unit tests pass (8 new covering slugify, grouping, collision guard, type grouping); tsc --noEmit and eslint clean.
• npm run build produces 217 prerendered /city/*.html pages; next start serves /city/%E5%8E%A6%E9%97%A8 (厦门), /city/mumbai, and /city/new_delhi with 200 and unknown slugs with 404.

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