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/alembic/versions/052_media_browser_indexes.py

@@ -0,0 +1,31 @@
+"""Add indexes for media browser filtering.
+
+Revision ID: 052
+Revises: 051
+"""
+from alembic import op
+
+revision = "052"
+down_revision = "051"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Index for filtering media_assets by asset_type + created_at ordering
+    op.create_index(
+        "ix_media_assets_asset_type_created",
+        "media_assets",
+        ["asset_type", "created_at"],
+    )
+    # Index for filtering products by category + lagertyp
+    op.create_index(
+        "ix_products_category_lagertyp",
+        "products",
+        ["category_key", "lagertyp"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_products_category_lagertyp", table_name="products")
+    op.drop_index("ix_media_assets_asset_type_created", table_name="media_assets")