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/material_service.py

@@ -12,7 +12,6 @@ from app.models.catalog import (
     BrandMaterial,
     Material,
     MaterialAttributeValue,
-    MaterialCategory,
     Unit,
 )
 from app.schemas.catalog import MaterialAttributeIn, MaterialCreate, MaterialUpdate
@@ -110,9 +109,7 @@ async def validate_attributes(
 async def create_material(db: AsyncSession, payload: MaterialCreate) -> Material:
     """Create a material + brand_materials rows + EAV values in ONE transaction.
 
-    v2.5/2.6: category_id is OPTIONAL — a global material may be created
-    without a category (it gets derived automatically from the first linked
-    supplier). name is GLOBALLY unique.
+    v2.6/v2.7: materials are global and category-free. name is GLOBALLY unique.
     """
     async with db.begin():
         duplicate = await db.scalar(
@@ -121,14 +118,10 @@ async def create_material(db: AsyncSession, payload: MaterialCreate) -> Material
         if duplicate is not None:
             raise ConflictError("متریالی با این نام در کل کاتالوگ قبلاً ثبت شده است", field="name")
 
-        if payload.category_id is not None:
-            if await db.get(MaterialCategory, payload.category_id) is None:
-                raise NotFoundError("دسته‌بندی یافت نشد", field="category_id")
         if not await db.get(Unit, payload.canonical_unit_id):
             raise NotFoundError("واحد مرجع یافت نشد", field="canonical_unit_id")
 
         material = Material(
-            category_id=payload.category_id,
             name=payload.name,
             topic=payload.topic,
             canonical_unit_id=payload.canonical_unit_id,