Add tests for background tasks and fail2ban client utility

- tests/test_tasks/test_blocklist_import.py: 14 tests, 96% coverage
- tests/test_tasks/test_health_check.py: 12 tests, 100% coverage
- tests/test_tasks/test_geo_cache_flush.py: 8 tests, 100% coverage
- tests/test_services/test_fail2ban_client.py: 24 new tests, 96% coverage

Total: 50 new tests (628 → 678 passing). Overall coverage 85% → 87%.
ruff, mypy --strict, tsc, and eslint all clean.

backend/tests/test_tasks/test_geo_cache_flush.py

@@ -0,0 +1,136 @@
+"""Tests for the geo cache flush background task.
+
+Validates that :func:`~app.tasks.geo_cache_flush._run_flush` correctly
+delegates to :func:`~app.services.geo_service.flush_dirty` and only logs
+when entries were actually flushed, and that
+:func:`~app.tasks.geo_cache_flush.register` configures the APScheduler job
+with the correct interval and stable job ID.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from app.tasks.geo_cache_flush import GEO_FLUSH_INTERVAL, JOB_ID, _run_flush, register
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_app(flush_count: int = 0) -> MagicMock:
+    """Build a minimal mock ``app`` for geo cache flush task tests.
+
+    Args:
+        flush_count: The value returned by the mocked ``flush_dirty`` call.
+
+    Returns:
+        A :class:`unittest.mock.MagicMock` that mimics ``fastapi.FastAPI``.
+    """
+    app = MagicMock()
+    app.state.db = MagicMock()
+    app.state.scheduler = MagicMock()
+    return app
+
+
+# ---------------------------------------------------------------------------
+# Tests for _run_flush
+# ---------------------------------------------------------------------------
+
+
+class TestRunFlush:
+    """Tests for :func:`~app.tasks.geo_cache_flush._run_flush`."""
+
+    @pytest.mark.asyncio
+    async def test_run_flush_calls_flush_dirty_with_db(self) -> None:
+        """``_run_flush`` must call ``geo_service.flush_dirty`` with ``app.state.db``."""
+        app = _make_app()
+
+        with patch(
+            "app.tasks.geo_cache_flush.geo_service.flush_dirty",
+            new_callable=AsyncMock,
+            return_value=0,
+        ) as mock_flush:
+            await _run_flush(app)
+
+        mock_flush.assert_awaited_once_with(app.state.db)
+
+    @pytest.mark.asyncio
+    async def test_run_flush_logs_when_entries_flushed(self) -> None:
+        """``_run_flush`` must emit a debug log when ``flush_dirty`` returns > 0."""
+        app = _make_app()
+
+        with patch(
+            "app.tasks.geo_cache_flush.geo_service.flush_dirty",
+            new_callable=AsyncMock,
+            return_value=15,
+        ), patch("app.tasks.geo_cache_flush.log") as mock_log:
+            await _run_flush(app)
+
+        debug_calls = [c for c in mock_log.debug.call_args_list if c[0][0] == "geo_cache_flush_ran"]
+        assert len(debug_calls) == 1
+        assert debug_calls[0][1]["flushed"] == 15
+
+    @pytest.mark.asyncio
+    async def test_run_flush_does_not_log_when_nothing_to_flush(self) -> None:
+        """``_run_flush`` must not emit any log when ``flush_dirty`` returns 0."""
+        app = _make_app()
+
+        with patch(
+            "app.tasks.geo_cache_flush.geo_service.flush_dirty",
+            new_callable=AsyncMock,
+            return_value=0,
+        ), patch("app.tasks.geo_cache_flush.log") as mock_log:
+            await _run_flush(app)
+
+        debug_calls = [c for c in mock_log.debug.call_args_list if c[0][0] == "geo_cache_flush_ran"]
+        assert debug_calls == []
+
+
+# ---------------------------------------------------------------------------
+# Tests for register
+# ---------------------------------------------------------------------------
+
+
+class TestRegister:
+    """Tests for :func:`~app.tasks.geo_cache_flush.register`."""
+
+    def test_register_adds_interval_job_to_scheduler(self) -> None:
+        """``register`` must add a job with an ``"interval"`` trigger."""
+        app = _make_app()
+
+        register(app)
+
+        app.state.scheduler.add_job.assert_called_once()
+        _, kwargs = app.state.scheduler.add_job.call_args
+        assert kwargs["trigger"] == "interval"
+        assert kwargs["seconds"] == GEO_FLUSH_INTERVAL
+
+    def test_register_uses_stable_job_id(self) -> None:
+        """``register`` must use the module-level ``JOB_ID`` constant."""
+        app = _make_app()
+
+        register(app)
+
+        _, kwargs = app.state.scheduler.add_job.call_args
+        assert kwargs["id"] == JOB_ID
+
+    def test_register_sets_replace_existing(self) -> None:
+        """``register`` must use ``replace_existing=True`` to avoid duplicate jobs."""
+        app = _make_app()
+
+        register(app)
+
+        _, kwargs = app.state.scheduler.add_job.call_args
+        assert kwargs["replace_existing"] is True
+
+    def test_register_passes_app_in_kwargs(self) -> None:
+        """The scheduled job must receive ``app`` as a kwarg for state access."""
+        app = _make_app()
+
+        register(app)
+
+        _, kwargs = app.state.scheduler.add_job.call_args
+        assert kwargs["kwargs"] == {"app": app}