feat(versions): prune_auto_pins FIFO-trims auto-pinned bucket

Auto-pinned versions live in their own bucket with MAX_AUTO_PINS=25 cap.
The scan job calls this after each note's promotions complete; the
oldest auto-pinned rows are dropped past the cap. Manual pins and
rolling rows are untouched.

tests/test_version_pinning_prune.py

@@ -43,3 +43,41 @@ async def test_create_version_prune_sql_filters_to_unpinned():
     assert any("pin_kind IS NULL" in s for s in captured_sql), (
         f"Expected prune SQL to filter pin_kind IS NULL; got: {captured_sql!r}"
     )
+
+
+async def test_prune_auto_pins_filters_to_auto_kind():
+    """prune_auto_pins must filter to pin_kind='auto' so manual pins and
+    rolling rows aren't touched, and must bind MAX_AUTO_PINS as the OFFSET."""
+    mock_session = AsyncMock()
+    mock_session.commit = AsyncMock()
+    mock_session.__aenter__ = AsyncMock(return_value=mock_session)
+    mock_session.__aexit__ = AsyncMock(return_value=False)
+
+    captured_sql: list[str] = []
+    captured_params: list[dict] = []
+
+    async def execute_capture(stmt, *args, **kwargs):
+        captured_sql.append(str(stmt))
+        try:
+            captured_params.append(dict(stmt.compile().params))
+        except Exception:
+            captured_params.append({})
+        return MagicMock()
+
+    mock_session.execute = AsyncMock(side_effect=execute_capture)
+
+    with patch(
+        "fabledassistant.services.version_pinning.async_session",
+        return_value=mock_session,
+    ):
+        from fabledassistant.services.version_pinning import (
+            prune_auto_pins, MAX_AUTO_PINS,
+        )
+        await prune_auto_pins(user_id=1, note_id=42)
+
+    assert any("pin_kind = 'auto'" in s for s in captured_sql), (
+        f"expected auto-bucket filter in SQL; got: {captured_sql!r}"
+    )
+    assert any(
+        p.get("max_auto_pins") == MAX_AUTO_PINS for p in captured_params
+    ), f"bound params: {captured_params!r}"