data: city-slug convention violations in the dataset ("厦门", "new delhi", "philadephia") + validator doesn't enforce slugs

[shauryagangrade]

Problem

The documented convention (src/lib/types.ts:12-17, data/places.schema.json:24-27) is lowercase snake_case city slugs, but the dataset contains violations the validator never catches:

• data/places/library.json — "city": "厦门" (non-CJK-unfriendly: becomes percent-encoded URLs via city-pages.ts:71-74)
• data/places/sat_centre.json — "city": "new delhi" ×3 (literal space, breaks slug generation)
• data/places/sat_centre.json — "philadephia" (misspelled Philadelphia → wrong city page)

Fix

1. Correct the records to canonical slugs (xiamen, new_delhi, philadelphia)
2. Extend scripts/validate-places.mjs to enforce the slug pattern /^[a-z0-9]+(_[a-z0-9]+)*$/ so these can't regress
3. Check for any other non-conforming values while in there (jq -r '.[].city' data/places/*.json | sort -u)

Acceptance criteria

• All cities conform; npm run validate enforces it