Fix crash on -a --self-check by adding exception handling to site check coroutines (#2466)

* Initial plan

* Fix crash on -a --self-check by adding exception handling in site_self_check and self_check

Wrap the body of site_self_check in try/except to catch unexpected errors
and always return a valid changes dict. Also add a safety-net try/except
in self_check around awaiting individual site check futures so that a
single site failure doesn't crash the entire self-check process.

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Co-authored-by: soxoj <31013580+soxoj@users.noreply.github.com>

* Restore @pytest.mark.slow on test_maigret_results

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Co-authored-by: soxoj <31013580+soxoj@users.noreply.github.com>

* Document --self-check error resilience, --auto-disable, and --diagnose in docs/

Update command-line-options.rst with expanded --self-check description
and new --auto-disable and --diagnose entries. Add a "Database self-check"
section to features.rst explaining error-resilient behaviour and usage
examples. Update usage-examples.rst to reference --auto-disable.

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: soxoj <31013580+soxoj@users.noreply.github.com>

tests/test_maigret.py

@@ -12,7 +12,8 @@ from maigret.maigret import (
     extract_ids_from_page,
     extract_ids_from_results,
 )
-from maigret.sites import MaigretSite
+from maigret.checking import site_self_check
+from maigret.sites import MaigretSite, MaigretDatabase
 from maigret.result import MaigretCheckResult, MaigretCheckStatus
 from tests.conftest import RESULTS_EXAMPLE
 
@@ -83,6 +84,41 @@ async def test_self_check_progressbar_enabled_by_default(test_db):
         assert kwargs.get('disable') is False
 
 
+@pytest.mark.asyncio
+async def test_site_self_check_handles_exception(test_db):
+    """Verify that site_self_check catches unexpected exceptions and returns a valid result."""
+    logger = Mock()
+    sem = asyncio.Semaphore(1)
+    site = test_db.sites_dict['ValidActive']
+
+    with patch('maigret.checking.maigret', side_effect=RuntimeError("test crash")):
+        result = await site_self_check(site, logger, sem, test_db)
+
+    assert isinstance(result, dict)
+    assert "issues" in result
+    assert len(result["issues"]) > 0
+    assert any("Unexpected error" in issue for issue in result["issues"])
+
+
+@pytest.mark.asyncio
+async def test_self_check_handles_task_exception(test_db):
+    """Verify that self_check continues when individual site checks raise exceptions."""
+    logger = Mock()
+
+    with patch('maigret.checking.maigret', side_effect=RuntimeError("test crash")):
+        result = await self_check(
+            test_db, test_db.sites_dict, logger, silent=True,
+            no_progressbar=True,
+        )
+
+    assert isinstance(result, dict)
+    assert 'results' in result
+    assert len(result['results']) == len(test_db.sites_dict)
+    for r in result['results']:
+        assert 'site_name' in r
+        assert 'issues' in r
+
+
 @pytest.mark.slow
 @pytest.mark.skip(reason="broken, fixme")
 def test_maigret_results(test_db):