refactor(catalog)!: decouple materials from categories (v2.7)

Materials are now global and category-free. The category lives only on the supplier (suppliers.category_id); every category-facing query goes through the supplier edge instead of the material row.

Schema (migration.sql + init_db DDL):
- drop the materials.category_id column, its FK, and the v2.5 first-link-wins automation (fn_derive_material_category + trg_supplier_materials_derive_category) — the DDL runs idempotently at API startup

Backend:
- GET /materials?category_id=… now filters via EXISTS(supplier_materials ⋈ suppliers.category_id)
- quote category filters (quotes list) use the quote's supplier category
- /pm/on-the-way derives the display category from the supplier of the quote
- supplier_materials links no longer derive/return a material category
- expand_service: material expand=category resolves to null (kept for contract compatibility)
- material create/update no longer validates or stores category_id; seed.py aligned

Frontend:
- material creation no longer sends category_id
- formatMaterialTitle reworked to a strict order with « . » separator: [Brand] . [Material] . [Spec 1] . [Spec 2], brand deduped when the stored name already starts with it; specs separator switched from « · » to « . »

Validated: python compile, api + frontend image builds (tsc + vite) clean; branch cut from the tip of prod_v4 (= origin/production, 5a9ef40 — the team's merge made them identical).

backend/app/db/init_db.py

@@ -379,16 +379,16 @@ async def ensure_v26(engine: AsyncEngine) -> None:
             "VALUES (:code, :name_fa, :dimension, :si_factor, :is_base) ON CONFLICT (code) DO NOTHING",
             u,
         )
-    # v2.5 automation triggers (idempotent — DROP + CREATE keeps them in sync).
+    # v2.5/v2.6 category decoupling from materials: drop trigger, function, and column.
     await run_statement("DROP TRIGGER IF EXISTS trg_supplier_materials_derive_category ON supplier_materials")
-    await run_statement("""
-        CREATE TRIGGER trg_supplier_materials_derive_category
-            AFTER INSERT ON supplier_materials
-            FOR EACH ROW EXECUTE FUNCTION fn_derive_material_category()
-    """)
+    await run_statement("DROP FUNCTION IF EXISTS fn_derive_material_category()")
+    await run_statement("ALTER TABLE materials DROP CONSTRAINT IF EXISTS materials_category_id_fkey")
+    await run_statement("ALTER TABLE materials DROP COLUMN IF EXISTS category_id")
+
     await run_statement("DROP TRIGGER IF EXISTS trg_item_decisions_sync_price ON item_decisions")
     await run_statement("""
         CREATE TRIGGER trg_item_decisions_sync_price
             AFTER INSERT OR UPDATE OF status ON item_decisions
             FOR EACH ROW EXECUTE FUNCTION fn_sync_last_quoted_price()
     """)
+