fix: remove EXIT trap and fix $USER expansion in DATABASE_URL

- Remove 'trap ... EXIT' that kills services when entering the dev shell
- Fix DATABASE_URL: $USER → $USER (no backslash) so bash expands it
  instead of passing the literal string 'USER' as the PG role name

nix/shell.nix

@@ -264,7 +264,7 @@ mkShell {
     export PGHOST="${pgHost}"
     export PGPORT="${pgPort}"
     export PGUSER="$USER"
-    export DATABASE_URL="ecto://\$USER@${pgHost}:${pgPort}/towerops_dev"
+    export DATABASE_URL="ecto://$USER@${pgHost}:${pgPort}/towerops_dev"
 
     # Redis connection
     export REDIS_URL="redis://${pgHost}:${redisPort}/0"
@@ -296,9 +296,6 @@ mkShell {
       fi
     fi
 
-    # Register trap to stop services on shell exit
-    trap '${stopServices}/bin/stop-services' EXIT
-
     # Install pre-commit hooks
     ${pre-commit-check.shellHook}