Permissionless city HINT for the shared client location resolver.

Owner ruling 2026-08-18: no public surface may prompt for browser geolocation on load. The client infers a city from its own IANA timezone and calls this endpoint only to REFINE that guess (a timezone like Asia/Kolkata covers all of India). The client cross-checks tz against its own timezone and discards the hint on mismatch, so a NATed/VPN answer degrades to the timezone-derived city instead of relocating the visitor.

available: false is a normal, non-error answer. Visitor IPs are used transiently — never persisted, never logged (see app/ip_location.py).