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/README.md

@@ -123,6 +123,7 @@ rules:
 | `missing_primary_key` | `have_primary_key` | Ensure tables have primary keys |
 | `orphaned_foreign_key` | `orphaned_foreign_key` | Detect FKs referencing non-existent tables |
 | `circular_dependency` | `circular_dependency` | Detect circular FK dependencies |
+| `duplicate_index_name` | `duplicate_index_name` | Index / PK / unique names must be unique per schema (default: `enforce`) |
 
 ## Rule Configuration