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/tools/seed.py

@@ -261,9 +261,7 @@ async def run() -> None:
         material_by_name: dict[str, Material] = {}
         brand_material_by_material: dict[str, object] = {}
         for name, cat_slug, unit_code, brand_name in MATERIALS:
-            cat = await get_category(db, cat_slug)
             material = Material(
-                category_id=cat.id,
                 name=name,
                 canonical_unit_id=unit_ids[unit_code],
                 status="active",
@@ -289,8 +287,10 @@ async def run() -> None:
             await db.flush()
             suppliers.append(sup)
         # last-quoted prices: link each supplier to 2 materials of its category
+        sup_cat_slug = {name: cat_slug for name, cat_slug, *_ in SUPPLIERS}
         for sup in suppliers:
-            cat_materials = [m for m in material_by_name.values() if m.category_id == sup.category_id]
+            target_slug = sup_cat_slug.get(sup.name)
+            cat_materials = [material_by_name[mat_name] for mat_name, c_slug, *_ in MATERIALS if c_slug == target_slug]
             for m in cat_materials[:2]:
                 db.add(SupplierMaterial(
                     supplier_id=sup.id, material_id=m.id, is_active=True,