fix: auto-set DAV_SKIP_INIT=1 in test init to match local GO_TESTING behavior

The DAV_SKIP_INIT env var was only set in CI, not locally. db_test_init.go always auto-sets GO_TESTING=1 for local runs — now does the same for DAV_SKIP_INIT, so tests don't panic on DAV init failure.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/db/db_test_init.go

@@ -30,4 +30,7 @@ func init() {
 	if os.Getenv("GO_TESTING") == "" {
 		os.Setenv("GO_TESTING", "1")
 	}
+	if os.Getenv("DAV_SKIP_INIT") == "" {
+		os.Setenv("DAV_SKIP_INIT", "1")
+	}
 }