feat(phase7.2): media browser with server-side filters + pagination

- Migration 052: indexes on media_assets(asset_type, created_at) and
  products(category_key, lagertyp) for efficient filter queries
- GET /api/media/assets: JOINs media_assets→products→order_lines,
  filters by asset_type / category_key / render_status / q (ILIKE),
  paginated (page/page_size), returns total+pages count
- New schemas: MediaAssetBrowseItem, MediaAssetBrowseResponse
- frontend/src/api/media.ts: getMediaAssets(filters), typed interfaces
- MediaBrowser.tsx: rewritten with sticky filter bar (debounced search,
  type/category/status dropdowns), responsive grid, image previews,
  download buttons, pagination footer with page size selector
- Renamed legacy function to listMediaAssets for backward compat

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/CadPreview.tsx

@@ -3,7 +3,7 @@ import { useParams, useNavigate } from 'react-router-dom'
 import { useQuery, useMutation } from '@tanstack/react-query'
 import { Box, Loader2, X } from 'lucide-react'
 import ThreeDViewer from '../components/cad/ThreeDViewer'
-import { getMediaAssets } from '../api/media'
+import { listMediaAssets as getMediaAssets } from '../api/media'
 import { generateGltfGeometry } from '../api/cad'
 
 /**