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/models/health_domain.py

@@ -0,0 +1,23 @@
+"""Health domain models.
+
+Internal domain-focused models used by health_service. These represent the
+business domain layer and are independent of HTTP response shapes.
+
+Response models are defined in `app.models.config` and mappers convert domain
+models to response models at the router boundary.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class DomainServerStatus:
+    """Cached fail2ban server health snapshot (domain model)."""
+
+    online: bool
+    version: str | None = None
+    active_jails: int = 0
+    total_bans: int = 0
+    total_failures: int = 0