fix(seed): enforce only the 8 canonical topic categories

The topic grids (PM system + wizard) showed extra categories like «سیم»,
«کابل», «ثبثب», «dsfg». Those rows were leftover junk in material_categories
from earlier testing — previously invisible because the frontend hardcoded
the 8-topic list; the API-driven topic grid surfaced them. The rows are
deleted (they had no references) and the seeder now deletes any category
outside the 8 canonical slugs, so a re-seed always yields exactly the same
8 categories as before. Dump refreshed from the clean state.

backend/tools/seed.py

@@ -212,6 +212,18 @@ async def run() -> None:
             "category_reference_units, unit_conversions, standard_conversions "
             "RESTART IDENTITY CASCADE"
         ))
+        # Only the 8 canonical topic categories may exist — any leftover slug
+        # (e.g. «سیم», «کابل» from earlier testing) is removed so the topic
+        # grids keep showing exactly the same 8 categories as always.
+        await db.execute(
+            text(
+                "DELETE FROM material_categories WHERE slug NOT IN ("
+                "'mechanical-installations-pipes','electrical-equipment-supplies',"
+                "'doors-windows-glass','metals-steel-sections','construction-materials',"
+                "'construction-chemicals','finishing-interior-decoration',"
+                "'construction-tools-equipment')"
+            )
+        )
 
         # ---- users (roles bound to Casdoor identities by username) ----
         users: dict[str, User] = {}