feat(backend): override create_schema_differ() to supply PostgresSchemaDiffer

The core migrated schema-diff resolution to the dependency-inversion hook
SQLDialectBase.create_schema_differ() and no longer imports concrete
backend differs. Override the hook so PostgreSQL keeps its specialized
schema comparison in migration snapshot diffs.

- Implement PostgresDialect.create_schema_differ() returning PostgresSchemaDiffer
- Add changelog fragment

Author: vistart

changelog.d/55.added.md

@@ -0,0 +1 @@
+Added `PostgresDialect.create_schema_differ()` returning `PostgresSchemaDiffer` so the dialect exposes its schema comparison through the core's new dependency-inversion hook (`SQLDialectBase.create_schema_differ()`), keeping migration snapshot diffs correct now that the core no longer imports concrete backend differs.

src/rhosocial/activerecord/backend/impl/postgres/dialect.py

@@ -640,6 +640,14 @@ def get_server_version(self) -> Tuple[int, int, int]:
         """Return the PostgreSQL version this dialect is configured for."""
         return self.version
 
+    def create_schema_differ(self):
+        """Return the PostgreSQL schema differ for this dialect."""
+        from rhosocial.activerecord.backend.impl.postgres.schema.differ import (
+            PostgresSchemaDiffer,
+        )
+
+        return PostgresSchemaDiffer()
+
     def format_datetime_diff_expression(self, expr: "Any") -> Tuple[str, Tuple]:
         start_sql, start_params = expr.start.to_sql()
         end_sql, end_params = expr.end.to_sql()