Problem

psql supports comma-separated host lists for failover:

psql -h host1,host2,host3 -p 5432 -U postgres

rpg does not. This is a psql parity gap that affects HA setups (Patroni, pgBouncer, etc.).

Spec

Follow libpq semantics exactly:

• -h host1,host2,host3 — try each in order, use first that accepts connection
• -p 5432,5433 — per-host ports (must have 1 port or N ports matching N hosts)
• URI form: postgres://host1:5432,host2:5432/db
• Connection string form: host=host1,host2 port=5432
• Stop at first successful connection
• Error message when all hosts fail: list which hosts were tried and why each failed

Reference

• libpq docs: https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-MULTIPLE-HOSTS
• tokio-postgres already parses multi-host connstrings via Config::from_str — leverage this

Implementation hints

• src/connection.rs: ConnParams currently stores single host/port — extend to Vec<(host, port)>
• connect() function needs retry loop over hosts
• Error reporting: collect per-host errors, report all if all fail
• Integration tests: at least 2 Docker containers, test failover (first down, second up)

Acceptance criteria

• rpg -h host1,host2 -U u -d db works (first host down → falls through to second)
• URI and connstring forms work
• Error lists all attempted hosts when all fail
• Integration tests cover: first-host-fails, all-fail, single-host (regression)
• Existing connection tests still pass