perf(webapp): scope env var create pre-check to submitted keys (#4579)

## Summary

Setting or importing environment variables ran a conflict pre-check that
loaded every variable in the project and every value across all of its
environments, only to decide whether the submitted keys already had a
value in the target environments. On projects with many variables and
environments that meant reading tens of thousands of rows on each
create/import call.

This scopes the pre-check to the submitted keys and target environments,
so it reads only the rows it actually inspects (submitted keys × target
envs), wrapped in `boundedIn` to keep the prepared-statement cache
stable. Same conflict detection, a handful of rows instead of the whole
project's env-var values.

.server-changes/scope-env-var-create-precheck.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Speed up setting and importing environment variables for projects with many variables.