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/routers/quotes.py

@@ -39,9 +39,8 @@ router = APIRouter(prefix="/quotes", tags=["quotes"])
 def _quote_category_exists(category_id) -> exists:
     return exists(
         select(1).where(
-            LineItem.quote_id == Quote.id,
-            Material.id == LineItem.material_id,
-            Material.category_id == category_id,
+            Supplier.id == Quote.supplier_id,
+            Supplier.category_id == category_id,
         )
     )