fix: Add explicit deps.get in configure phase

Override configurePhase to explicitly run 'mix deps.get' before
compilation. This ensures hex dependencies (like ecto) are available
when vendored path dependencies (oban_met) are compiled.

nix/build.nix

@@ -40,6 +40,16 @@ beamPackages.mixRelease {
   # Environment variables for build
   MIX_OS_DEPS_COMPILE_PARTITION_COUNT = "6";
 
+  # Override Mix phases to handle vendored deps properly
+  configurePhase = ''
+    runHook preConfigure
+
+    # Get all deps (including hex deps that vendored packages need)
+    mix deps.get --only prod
+
+    runHook postConfigure
+  '';
+
   # Pre-build: Copy pre-built NIF and stub the Makefile to skip rebuild
   preBuild = ''
     # Copy pre-built NIF shared library