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.

Agent-Logs-Url: https://github.com/soxoj/maigret/sessions/5e27d620-5cbb-43d2-a9f9-ecb53a29904d

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

* Restore @pytest.mark.slow on test_maigret_results

Agent-Logs-Url: https://github.com/soxoj/maigret/sessions/5e27d620-5cbb-43d2-a9f9-ecb53a29904d

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.

Agent-Logs-Url: https://github.com/soxoj/maigret/sessions/af1f0f09-9112-4902-8475-e81d235ff3ed

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

---------

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

docs/source/usage-examples.rst

@@ -33,7 +33,7 @@ Use Cases
    If you experience many false positives, you can do the following:
 
    - Install the last development version of Maigret from GitHub
-   - Run Maigret with ``--self-check`` flag and agree on disabling of problematic sites
+   - Run Maigret with ``--self-check --auto-disable`` flag and agree on disabling of problematic sites
 
 3. Search for accounts with username ``machine42`` and generate HTML and PDF reports.