fix(restore): drop user-owned collations in the --empty pre-clean

The drop_sql loop covered relations, routines, sequences and types but
not pg_collation, so a dump's CREATE COLLATION (OpenProject's ICU
public.versions_name) aborted the ON_ERROR_STOP replay with 'collation
already exists'. Add the fifth UNION ALL branch; DROP COLLATION IF
EXISTS public.<name> CASCADE rides the existing loop, and the loop
already drops every user table, so CASCADE fallout is absorbed by the
IF EXISTS no-ops.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit/restore/test_postgres_empty_drop.py

@@ -0,0 +1,43 @@
+"""The --empty pre-clean drop must cover every object class a dump can
+re-CREATE ahead of tables; a missed class aborts the replay under
+ON_ERROR_STOP (OpenProject's ICU collation public.versions_name did)."""
+
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+from baudolo.restore.db import postgres
+
+
+class TestPostgresEmptyDrop(unittest.TestCase):
+    def _drop_sql(self) -> str:
+        with tempfile.NamedTemporaryFile(suffix=".sql") as fh:
+            Path(fh.name).write_bytes(b"SELECT 1;\n")
+            with mock.patch.object(postgres, "docker_exec") as run:
+                postgres.restore_postgres_sql(
+                    container="c",
+                    db_name="db",
+                    user="u",
+                    password="p",
+                    sql_path=fh.name,
+                    empty=True,
+                )
+        first_call = run.call_args_list[0]
+        return first_call.kwargs["stdin"].decode()
+
+    def test_drop_covers_collations(self) -> None:
+        sql = self._drop_sql()
+        self.assertIn("pg_collation", sql)
+        self.assertIn("'COLLATION' AS type", sql)
+        self.assertIn("pg_get_userbyid(col.collowner) = current_user", sql)
+
+    def test_drop_still_covers_the_other_classes(self) -> None:
+        sql = self._drop_sql()
+        for marker in ("pg_class", "pg_proc", "'SEQUENCE' AS type", "'TYPE' AS type"):
+            self.assertIn(marker, sql)
+        self.assertIn("DROP %s IF EXISTS public.%I CASCADE", sql)
+
+
+if __name__ == "__main__":
+    unittest.main()