refactor(api): rename /filaments/ to /filament-catalog/ for clarity (#427)

The /filaments/ endpoint manages material type definitions (cost, temps,
density) but shares its name with the UI's "Filament" page which shows
the spool inventory (/inventory/spools/). This caused users to expect
the API to return their spool inventory.

Rename to /filament-catalog/ to make the distinction clear. No frontend
behavior change — these API methods are defined but unused in the UI.

frontend/src/api/client.ts

@@ -3312,10 +3312,10 @@ export const api = {
       { method: 'POST' }
     ),
 
-  // Filaments
-  listFilaments: () => request<Filament[]>('/filaments/'),
-  getFilament: (id: number) => request<Filament>(`/filaments/${id}`),
-  getFilamentsByType: (type: string) => request<Filament[]>(`/filaments/by-type/${type}`),
+  // Filament Catalog (material types with cost/temp data)
+  listFilaments: () => request<Filament[]>('/filament-catalog/'),
+  getFilament: (id: number) => request<Filament>(`/filament-catalog/${id}`),
+  getFilamentsByType: (type: string) => request<Filament[]>(`/filament-catalog/by-type/${type}`),
 
   // Notification Providers
   getNotificationProviders: () => request<NotificationProvider[]>('/notifications/'),