app: CRAP scores as a Lens sensor (ex_crap)

The coverage export from `test.coverage` now also feeds a `crap` sensor,
second in app's VERIFY_SENSORS. It runs with MIX_ENV=test so it reads the
export's own environment instead of compiling a dev build and scoring a
different module set, and it guards on the export's existence because
ex_crap silently falls back to 0% coverage when the file is missing.

Baseline at ex_crap's default threshold (30): 23 files, 139 functions,
worst score 20.00 (CoreComponents.table/1, 0% - unused upstream scaffold).
No offenders, so nothing to refactor or cover yet.

app/mix.exs

@@ -81,7 +81,8 @@ defmodule Firehose.MixProject do
       {:microprints,
        git: "https://gitea.apps.sustainabledelivery.com/QWAN/microprints-phoenix.git",
        branch: "main"},
-      {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
+      {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
+      {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false}
     ]
   end