seo: sitemap sets lastModified = new Date() on every build, destroying the lastmod signal

[shauryagangrade]

Problem

src/app/sitemap.ts:12,25 stamps every URL with lastModified: new Date(). Since every deploy claims every page changed, crawlers learn to ignore lastmod entirely — a wasted signal. Real dates are available:

• docs/changelog pages → latest changelog entry date
• city pages → newest verified.on among that city's places
• static routes → omit lastModified rather than lie

Acceptance criteria

• lastModified values are real and stable between unrelated deploys