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>

backend/app/domains/media/schemas.py

@@ -25,3 +25,31 @@ class MediaAssetOut(BaseModel):
     thumbnail_url: str | None = None
 
     model_config = {"from_attributes": True}
+
+
+class MediaAssetBrowseItem(BaseModel):
+    """Enriched asset item for the media browser endpoint."""
+    id: uuid.UUID
+    asset_type: MediaAssetType
+    file_path: str
+    file_size_bytes: int | None
+    mime_type: str | None
+    created_at: datetime
+    order_line_id: uuid.UUID | None
+    product_id: uuid.UUID | None
+    product_name: str | None
+    product_pim_id: str | None
+    category_key: str | None
+    render_status: str | None
+    download_url: str | None = None
+    thumbnail_url: str | None = None
+
+    model_config = {"from_attributes": True}
+
+
+class MediaAssetBrowseResponse(BaseModel):
+    items: list[MediaAssetBrowseItem]
+    total: int
+    page: int
+    page_size: int
+    pages: int