Fix HIGH priority issues: unbounded queries, rate limiting, health checks

Issue #3 - Unbounded Query Results (OOM):
- get_all_archived_history() now uses keyset pagination with bounded max_rows (50k default)
- Added 'id' field to records from get_archived_history() and get_archived_history_keyset()
- Protocol signature updated with page_size, max_rows, last_ban_id params

Issue #7 - Docker Health Check Fails:
- Added curl to Dockerfile.backend runtime image
- HEALTHCHECK now uses 'curl -f http://localhost:8000/api/health'
- compose.prod.yml: increased start_period to 40s, timeout to 10s
- Frontend healthcheck proxies to backend /api/health

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/routers/blocklist.py

@@ -34,6 +34,7 @@ from app.dependencies import (
     SettingsDep,
 )
 from app.exceptions import BadRequestError, BlocklistSourceNotFoundError
+from app.mappers import blocklist_mappers
 from app.models.blocklist import (
     BlocklistListResponse,
     BlocklistSource,
@@ -370,6 +371,7 @@ async def preview_blocklist(
         raise BlocklistSourceNotFoundError(source_id)
 
     try:
-        return await blocklist_service.preview_source(source.url, http_session)
+        domain_result = await blocklist_service.preview_source(source.url, http_session)
+        return blocklist_mappers.map_domain_preview_result_to_response(domain_result)
     except ValueError as exc:
         raise BadRequestError(f"Could not fetch blocklist: {exc}") from exc