Recurring vrat scan for a city/range.

Uses the ratified panchanga scan rulesets, not the broad festival registry: Ekadashi, Purnima, Amavasya, Pradosha, Sankashti, and Masik Shivaratri.

Transport: the response is gzip-compressed when the client advertises Accept-Encoding: gzip and the body clears the size threshold; the decompressed body is byte-identical to the plain JSON, and identity clients are unchanged. The payload is computed by _panchang_vrats_dict, which /panchang/month also calls internally (consuming the dict directly, so it is unaffected by the route's transport encoding).