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/pm.py

@@ -195,17 +195,11 @@ async def pm_on_the_way(
     # unit / category lookups for display
     unit_ids = {d.LineItem.unit_id for d in decisions}
     units = {u.id: u for u in (await db.execute(select(Unit).where(Unit.id.in_(unit_ids)))).scalars()}
-    mat_ids = {d.Material.id for d in decisions}
+    sup_cat_ids = {d.Supplier.category_id for d in decisions if d.Supplier.category_id}
     cats = {}
-    if mat_ids:
-        rows = (
-            await db.execute(
-                select(Material.id, MaterialCategory)
-                .join(MaterialCategory, MaterialCategory.id == Material.category_id)
-                .where(Material.id.in_(mat_ids))
-            )
-        ).all()
-        cats = {m_id: c for m_id, c in rows}
+    if sup_cat_ids:
+        rows = (await db.execute(select(MaterialCategory).where(MaterialCategory.id.in_(sup_cat_ids)))).scalars().all()
+        cats = {c.id: c for c in rows}
 
     out = []
     for d, li, q, m, sup, proj in decisions:
@@ -220,7 +214,7 @@ async def pm_on_the_way(
             "project": {"id": str(proj.id), "name": proj.name} if proj else None,
             "decided_at": to_iso(d.decided_at),
             "material": {"id": str(m.id), "name": m.name,
-                         "category": {"id": str(c.id), "name": c.name} if (c := cats.get(m.id)) else None},
+                         "category": {"id": str(c.id), "name": c.name} if (c := cats.get(sup.category_id)) else None},
             "unit": {"id": units[li.unit_id].id, "code": units[li.unit_id].code,
                      "name_fa": units[li.unit_id].name_fa} if li.unit_id in units else None,
             "ordered_qty": str(li.quantity),