feat(ci): test.sh(go test / flutter test)

Add scripts/ci/test.sh <server|client>: server runs `go test ./...`
inside golang:1.25 (mirrors ci.yml's go-server job docker invocation +
gomod/gobuild caches + GOPROXY from _env.sh); client runs `flutter test`
inside ghcr.io/cirruslabs/flutter:stable (mirrors ci.yml's flutter-client
job's pub-cache mount + test dirs). Wired into deploy-server.yml's Test
step already added in Task 4. Registers the script in ci.yml's shellcheck
file list.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

.gitea/workflows/ci.yml

@@ -53,7 +53,8 @@ jobs:
               /mnt/scripts/ci/deploy-site.sh \
               /mnt/scripts/ci/compile-backend.sh \
               /mnt/scripts/ci/release-server.sh \
-              /mnt/scripts/ci/deploy-server.sh
+              /mnt/scripts/ci/deploy-server.sh \
+              /mnt/scripts/ci/test.sh
 
   # ── Job 2: OpenAPI Sync Check ────────────────────────────────────────────
   openapi-check: