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.

src/fabledassistant/services/version_pinning.py

@@ -88,3 +88,32 @@ async def unpin_version(
         await session.commit()
         await session.refresh(version)
         return version
+
+
+async def prune_auto_pins(user_id: int, note_id: int) -> None:
+    """FIFO-prune the auto-pinned bucket for one note past MAX_AUTO_PINS.
+
+    Manual pins and rolling rows are untouched. Called by the scan job
+    after each note's auto-pin promotions finish.
+    """
+    async with async_session() as session:
+        await session.execute(
+            text(
+                """
+                DELETE FROM note_versions
+                WHERE id IN (
+                    SELECT id FROM note_versions
+                    WHERE note_id = :note_id
+                      AND user_id = :user_id
+                      AND pin_kind = 'auto'
+                    ORDER BY created_at DESC
+                    OFFSET :max_auto_pins
+                )
+                """
+            ).bindparams(
+                note_id=note_id,
+                user_id=user_id,
+                max_auto_pins=MAX_AUTO_PINS,
+            )
+        )
+        await session.commit()