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/services/expand_service.py

@@ -206,11 +206,6 @@ async def _load_material_maps(db: AsyncSession, materials: list[Material], expan
     material_ids = [m.id for m in materials]
     maps: dict = {"categories": {}, "units": {}, "brands": {}, "material_brands": {}, "mavs": {}, "attr_defs": {}}
 
-    if "category" in expand:
-        cat_ids = {m.category_id for m in materials}
-        rows = (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(cat_ids)))).scalars().all()
-        maps["categories"] = {c.id: c for c in rows}
-
     unit_ids: set[int] = set()
     if "canonical_unit" in expand:
         unit_ids |= {m.canonical_unit_id for m in materials}
@@ -258,9 +253,9 @@ def _material_to_dict(m: Material, expand: set[str], maps: dict) -> dict:
         "updated_at": to_iso(m.updated_at),
     }
     if "category" in expand:
-        d["category"] = category_brief(maps["categories"].get(m.category_id))
+        d["category"] = None
     else:
-        d["category_id"] = str(m.category_id)
+        d["category_id"] = None
     if "canonical_unit" in expand:
         d["canonical_unit"] = unit_to_dict(maps["units"].get(m.canonical_unit_id))
     else:
@@ -326,9 +321,6 @@ async def _load_line_item_data(db: AsyncSession, line_items: list[LineItem], exp
         material_ids = {li.material_id for li in line_items}
         materials = (await db.execute(select(Material).where(Material.id.in_(material_ids)))).scalars().all()
         data.materials = {m.id: m for m in materials}
-        cat_ids = {m.category_id for m in materials}
-        cats = (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(cat_ids)))).scalars().all()
-        data.categories = {c.id: c for c in cats}
         bms = (await db.execute(select(BrandMaterial).where(BrandMaterial.material_id.in_(material_ids)))).scalars().all()
         brand_ids: set[uuid.UUID] = set()
         for bm in bms:
@@ -450,7 +442,7 @@ def _line_item_expanded(li: LineItem, expand: set[str], data: _LineItemData, inc
             d["material"] = {
                 "id": str(m.id),
                 "name": m.name,
-                "category": category_brief(data.categories.get(m.category_id)),
+                "category": None,
                 "brands": [
                     {"id": str(bm.brand_id), "name": data.brands[bm.brand_id].name, "catalog_code": bm.catalog_code}
                     for bm in data.material_brands.get(m.id, [])