new(patroni): PostgreSQL HA template (Zalando)

[tannevaled]

Summary

• Add a new pantry recipe for Patroni, the de-facto orchestrator for PostgreSQL high-availability with etcd / Consul / ZooKeeper as the DCS backend.
• Pure-Python install staged into a sealed venv via bkpyvenv stage / bkpyvenv seal, exposing patroni and patronictl.
• Pinned python.org: ^3.9 (matches patroni's classifiers / current build matrix). postgresql.org: '*' is declared at runtime because patroni shells out to pg_ctl, pg_rewind, pg_basebackup, pg_controldata.

Notes

• The project moved from zalando/patroni to patroni/patroni (community-maintained fork). GitHub still redirects the old path, but the recipe pins the canonical patroni/patroni URL and version source.
• DCS dependencies (etcd / Consul / ZooKeeper) are intentionally left commented out — users choose one, and patroni installs the Python client libraries (python-etcd, kazoo, py-consul) via pip.

Test plan

• pkgx +patroni patroni --version matches the pinned version.
• patronictl --version matches.
• CI green on Linux + macOS.