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/api/media.ts

@@ -10,6 +10,53 @@ export type MediaAssetType =
   | 'gltf_production'
   | 'blend_production'
 
+// ── Media Browser (server-side filtered + paginated) ──────────────────────────
+
+export interface MediaAssetFilters {
+  asset_type?: string
+  category_key?: string
+  render_status?: string
+  q?: string
+  page?: number
+  page_size?: number
+}
+
+export interface MediaAssetItem {
+  id: string
+  asset_type: MediaAssetType
+  file_path: string
+  file_size_bytes: number | null
+  mime_type: string | null
+  created_at: string
+  order_line_id: string | null
+  product_id: string | null
+  product_name: string | null
+  product_pim_id: string | null
+  category_key: string | null
+  render_status: string | null
+  download_url: string | null
+  thumbnail_url: string | null
+}
+
+export interface MediaAssetListResponse {
+  items: MediaAssetItem[]
+  total: number
+  page: number
+  page_size: number
+  pages: number
+}
+
+export function getMediaAssets(filters: MediaAssetFilters = {}): Promise<MediaAssetListResponse> {
+  const params = new URLSearchParams()
+  if (filters.asset_type) params.set('asset_type', filters.asset_type)
+  if (filters.category_key) params.set('category_key', filters.category_key)
+  if (filters.render_status) params.set('render_status', filters.render_status)
+  if (filters.q) params.set('q', filters.q)
+  if (filters.page !== undefined) params.set('page', String(filters.page))
+  if (filters.page_size !== undefined) params.set('page_size', String(filters.page_size))
+  return api.get(`/media/assets?${params}`).then(r => r.data)
+}
+
 export interface MediaAsset {
   id: string
   tenant_id: string | null
@@ -42,7 +89,7 @@ export interface MediaFilter {
   sort_dir?: 'asc' | 'desc'
 }
 
-export const getMediaAssets = (filters: MediaFilter = {}): Promise<MediaAsset[]> => {
+export const listMediaAssets = (filters: MediaFilter = {}): Promise<MediaAsset[]> => {
   const params = new URLSearchParams()
   if (filters.product_id) params.set('product_id', filters.product_id)
   if (filters.order_line_id) params.set('order_line_id', filters.order_line_id)