Replace HYDRA_DBI with HYDRA_DATABASE_URL everywhere

[Ericson2314]

Hydra grew a second database-connection convention when the Rust queue runner arrived: it takes a standard postgres:// URL (db_url / HYDRA_DATABASE_URL), while the Perl and C++ services read HYDRA_DBI, a Perl DBI string. Converge on the URL as the single convention before more Rust services appear, rather than teaching each of them the legacy DBI syntax.

The two formats are equally expressive — the PostgreSQL URI scheme accepts every libpq keyword as a query parameter — so nothing is lost, and each consumer needs only a small change:

• Perl: Hydra::Model::DB converts the URL to a DBI DSN with URI::db (new dependency). See the comment there for the user/password caveat.

• C++ (hydra-evaluator): libpq accepts a postgres:// URL verbatim, so the DBI-to-conninfo translation code is deleted outright.

• NixOS module: the dbi option is replaced by dbUrl, and all services now get HYDRA_DATABASE_URL with a per-service application_name query parameter. Neither hydra.nixos.org nor staging ever set dbi, so the option's removal breaks no known deployment.

• The foreman dev scripts, test harness, and manual switch accordingly. The test harness already exported both variables; the HYDRA_DBI half is now dropped.

The Perl and C++ in-code default becomes postgres:///hydra, which is byte-for-byte equivalent to the old dbi:Pg:dbname=hydra; (local socket, OS-user auth). The NixOS module default matches the queue runner's explicit postgres://hydra@%2Frun%2Fpostgresql:5432/hydra.