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).

sql/migration.sql

@@ -164,14 +164,12 @@ CREATE TABLE category_attributes (
     CONSTRAINT uq_category_attribute UNIQUE (category_id, attribute_definition_id)
 );
 
--- ---------- MATERIALS (GLOBAL catalog entities — v2.5) ----------
--- category_id is NULLABLE and DERIVED automatically from the first linked
--- supplier (supplier_materials insert, first-link-wins). name is GLOBALLY
+-- ---------- MATERIALS (GLOBAL catalog entities — v2.6/v2.7) ----------
+-- Materials are global, category-free entities. name is GLOBALLY
 -- unique (uq_material_name). topic is a legacy display column no longer
 -- written by the app.
 CREATE TABLE materials (
     id                UUID            NOT NULL DEFAULT gen_random_uuid() PRIMARY KEY,
-    category_id       UUID            REFERENCES material_categories(id) ON DELETE RESTRICT,
     name              TEXT            NOT NULL,
     topic             TEXT,           -- LEGACY display column
     canonical_unit_id INTEGER         NOT NULL REFERENCES units(id) ON DELETE RESTRICT,
@@ -585,24 +583,6 @@ CREATE TRIGGER trg_quote_attachments_updated_at
 -- AUTOMATION TRIGGERS (v2.5 + v2.6)
 -- ============================================================================
 
--- Derive materials.category_id from the FIRST linked supplier (first-link-wins).
--- Idempotent: only touches rows where category_id IS NULL.
-CREATE OR REPLACE FUNCTION fn_derive_material_category()
-RETURNS TRIGGER AS $$
-BEGIN
-    UPDATE materials
-    SET category_id = (SELECT category_id FROM suppliers WHERE id = NEW.supplier_id),
-        updated_at  = now()
-    WHERE id = NEW.material_id
-      AND category_id IS NULL;
-    RETURN NEW;
-END;
-$$ LANGUAGE plpgsql;
-
-CREATE TRIGGER trg_supplier_materials_derive_category
-    AFTER INSERT ON supplier_materials
-    FOR EACH ROW EXECUTE FUNCTION fn_derive_material_category();
-
 -- Sync supplier_materials.last_quoted_price / last_quoted_at when an item
 -- decision becomes 'chosen' (INSERT or UPDATE — the accept-quote flow
 -- inserts decisions directly as 'chosen').