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/models/catalog.py

@@ -130,14 +130,11 @@ class CategoryAttribute(Base):
 
 class Material(Base):
     __tablename__ = "materials"
-    # v2.5/2.6: name is GLOBALLY unique; category_id is nullable + derived.
+    # v2.6/v2.7: name is GLOBALLY unique; decoupled from categories.
     __table_args__ = (UniqueConstraint("name", name="uq_material_name"),)
     __mapper_args__ = {"eager_defaults": True}
 
     id: Mapped[uuid.UUID] = _uuid_pk()
-    category_id: Mapped[uuid.UUID | None] = mapped_column(
-        ForeignKey("material_categories.id", ondelete="RESTRICT")
-    )
     name: Mapped[str] = mapped_column(Text, nullable=False)
     topic: Mapped[str | None] = mapped_column(Text)  # تاپیک جلوی برنامه (Persian) — legacy
     canonical_unit_id: Mapped[int] = mapped_column(
@@ -149,7 +146,6 @@ class Material(Base):
         DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
     )
 
-    category: Mapped["MaterialCategory | None"] = relationship(lazy="raise")
     canonical_unit: Mapped["Unit"] = relationship(lazy="raise")
     attribute_values: Mapped[list["MaterialAttributeValue"]] = relationship(
         back_populates="material", lazy="raise", passive_deletes=True