Wire up Microwaveprop.AprsRepo as an optional secondary Ecto repo that
points at the sister aprs.me Postgres for read-only access. This is the
foundation for upcoming 144 MHz calibration work that uses verified APRS
RF hops as ground truth.
* lib/microwaveprop/aprs_repo.ex declares a read_only repo; no schemas,
no migrations, no writes — Microwaveprop never persists APRS data.
* Application supervisor starts the repo only when configured (dev/test
via config blocks, prod via APRS_DATABASE_URL). A missing config logs
a warning and skips startup so the main app still boots.
* config/dev.exs + config/test.exs add local aprsme_dev / aprsme_test
endpoints. Test pool uses Ecto SQL Sandbox; tests that don't touch
APRS data don't need to check the connection out.
* config/runtime.exs guards the prod block on APRS_DATABASE_URL —
unset env logs a warning and leaves the repo unconfigured.
