fix(ci): downgrade golangci-lint v2.11 to v2.9 to fix lint timeout

v2.10.1 introduced a recursive markDepsForAnalyzingSource change that
causes all transitive dependencies (including 132K lines of ent
generated code) to be analyzed from source instead of compiled export
data, leading to >30 min CI timeout.

v2.9.0 is the first version with Go 1.26 support (PR #6271, merged
Feb 10 2026) and does not have this performance regression.

.github/workflows/backend-ci.yml

@@ -42,6 +42,6 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: v2.11
+          version: v2.9
           args: --timeout=30m
           working-directory: backend