feat(inspector): fail on duplicate index names per schema

PostgreSQL index names share the schema-wide relation namespace, so a
reused name makes CREATE INDEX IF NOT EXISTS silently skip the duplicate.
Add duplicate_index_name rule (enforce by default) covering indexes and
PK/unique constraint backing indexes.

pkg/inspector/rules_test.go

@@ -37,6 +37,7 @@ func TestGetDefaultConfig(t *testing.T) {
 		"missing_primary_key",
 		"orphaned_foreign_key",
 		"circular_dependency",
+		"duplicate_index_name",
 	}
 
 	for _, ruleName := range expectedRules {