feat(catalog): auto-purge orphan materials when quotes/items release them

A material that ends up referenced by nothing (no line_items anywhere, no warehouse stock) was previously left behind forever, accumulating dead catalog rows as offers were edited or deleted.

- new quote_service.cleanup_orphan_materials(db, material_ids): for each candidate it checks usage across line_items (all quotes) and warehouse inventory; only when both are zero does it remove the auxiliary rows (supplier_materials, brand_materials, material_units, material_attribute_values) and finally the materials row; returns the purged ids
- replace_quote_children (edit modal): snapshots the quote's material ids before the children are replaced, diffs against the incoming items, and purges whatever was detached
- delete_quote: collects the quote's material ids + attachment keys first, deletes the quote, purges orphans inside the transaction, and only then (post-commit) removes the MinIO objects — now both the recorded attachment keys and a recursive remove_prefix(quote_id/) sweep, so stray objects under the quote folder cannot survive; the response's cascaded block reports purged_orphan_materials
- line_items: PATCH that switches a line item's material, and DELETE of a line item, both clean up the previously referenced material
- storage.remove_prefix(prefix): best-effort recursive object deletion under a prefix (sync MinIO client wrapped in asyncio.to_thread, same pattern as remove_object)

Safety: usage checks run against the whole system before any delete, so shared materials — including those held only by warehouse stock — are never touched. Verified: python compile + api image build clean.

backend/app/routers/line_items.py

@@ -108,6 +108,7 @@ async def update_line_item(
         effective_bm = payload.brand_material_id if payload.brand_material_id is not None else li.brand_material_id
         await ensure_brand_material_belong(db, effective_bm, new_material_id)
 
+    old_material_id = li.material_id
     if payload.material_id is not None:
         li.material_id = payload.material_id
         # v2.5: keep the supplier<->material catalog edge in sync.
@@ -121,6 +122,11 @@ async def update_line_item(
     if payload.notes is not None:
         li.notes = payload.notes
     await db.flush()
+
+    if payload.material_id is not None and payload.material_id != old_material_id:
+        from app.services.quote_service import cleanup_orphan_materials
+        await cleanup_orphan_materials(db, [old_material_id])
+
     return {"data": (await serialize_line_items(db, [li], set()))[0]}
 
 
@@ -133,6 +139,13 @@ async def delete_line_item(quote_id, line_item_id, db: AsyncSession = Depends(ge
     decision = await db.scalar(select(ItemDecision).where(ItemDecision.line_item_id == li.id))
     if decision is not None and decision.status == "chosen":
         raise ConflictError("خط آیتم انتخاب‌شده قابل حذف نیست")
+    old_material_id = li.material_id
     await db.delete(li)  # specs / links / decision / receipt cascade via DDL
     await db.flush()
+
+    if old_material_id:
+        from app.services.quote_service import cleanup_orphan_materials
+        await cleanup_orphan_materials(db, [old_material_id])
+
     return {"data": {"id": str(line_item_id), "deleted": True}}
+