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/inspector.go

@@ -161,6 +161,7 @@ func getValidator(functionName string) (validatorFunc, bool) {
 		"have_primary_key":              validateMissingPrimaryKey,
 		"orphaned_foreign_key":          validateOrphanedForeignKey,
 		"circular_dependency":           validateCircularDependency,
+		"duplicate_index_name":          validateDuplicateIndexName,
 	}
 
 	fn, exists := validators[functionName]