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/materials.py

@@ -18,7 +18,7 @@ from app.models.catalog import (
     MaterialUnit,
     Unit,
 )
-from app.models.procurement import LineItem
+from app.models.procurement import LineItem, Supplier, SupplierMaterial
 from app.schemas.catalog import (
     MaterialAttributeValuesPut,
     MaterialCreate,
@@ -108,7 +108,16 @@ async def list_materials(
 ):
     stmt = select(Material)
     if category_id:
-        stmt = stmt.where(Material.category_id == category_id)
+        sub = (
+            select(1)
+            .select_from(SupplierMaterial)
+            .join(Supplier, Supplier.id == SupplierMaterial.supplier_id)
+            .where(
+                SupplierMaterial.material_id == Material.id,
+                Supplier.category_id == category_id,
+            )
+        )
+        stmt = stmt.where(exists(sub))
     if brand_id:
         # WHERE EXISTS brand_materials with that brand_id
         sub = select(1).where(