feat: sitemap hreflang alternates, caching, and lastmod cleanup

Extract sitemap generation to sitemap.py with xhtml:link hreflang
alternates (en/de/x-default) on every URL entry. Add 1-hour in-memory
TTL cache with Cache-Control header. Include supplier pages in both
languages (were EN-only). Drop misleading "today" lastmod from static
pages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -21,6 +21,12 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - Supervisor: added daily sleep interval between pipeline runs
 
 ### Added
+- **Sitemap: hreflang alternates + caching** — extracted sitemap generation to
+  `sitemap.py`; each URL entry now includes `xhtml:link` hreflang alternates
+  (en, de, x-default) for correct international SEO signaling; supplier detail
+  pages now listed in both EN and DE (were EN-only); removed misleading "today"
+  lastmod from static pages; added 1-hour in-memory TTL cache with
+  `Cache-Control: public, max-age=3600` response header
 - **Playtomic availability extractor** (`playtomic_availability.py`) — daily next-day booking
   slot snapshots for occupancy rate estimation and pricing benchmarking; reads tenant IDs from
   latest `tenants.json.gz`, queries `/v1/availability` per venue with 2s throttle, resumable