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/supplier_materials.py

@@ -58,11 +58,7 @@ async def list_supplier_materials(
     items, meta = await paginate(stmt, page, page_size, db)
     mat_ids = {l.material_id for l in items}
     materials = {m.id: m for m in (await db.execute(select(Material).where(Material.id.in_(mat_ids)))).scalars()}
-    cat_ids = {m.category_id for m in materials.values() if m.category_id}
-    cats = {}
-    if cat_ids:
-        cats = {c.id: c for c in (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(cat_ids)))).scalars()}
-    return {"data": [link_to_dict(l, materials.get(l.material_id), cats.get(materials.get(l.material_id).category_id if l.material_id in materials else None)) for l in items], "meta": meta}
+    return {"data": [link_to_dict(l, materials.get(l.material_id), None) for l in items], "meta": meta}
 
 
 @router.post("/suppliers/{supplier_id}/materials", status_code=201)
@@ -72,8 +68,7 @@ async def create_supplier_material(
     async with db.begin():
         link = await link_supplier_material(db, supplier_id, payload.material_id)
         material = await db.get(Material, payload.material_id)
-        category = await db.get(MaterialCategory, material.category_id) if material and material.category_id else None
-    return {"data": link_to_dict(link, material, category)}
+    return {"data": link_to_dict(link, material, None)}
 
 
 @router.delete("/suppliers/{supplier_id}/materials/{material_id}")