One key for audit retention, and stop the SAML page claiming a feature exists

Wave 6: two settings pages that told an administrator the site was configured
when it was not. That is worse than a missing feature, because nobody goes
looking for it.

Audit retention had TWO keys. shopdb/core/services/auditretention.py reads
`audit_retentiondays` (category site, default 0). A second `audit_retention_days`
was seeded under category audit with a default of 90, and the Audit & Logging
page wrote to that one - so an administrator set a period, reloaded, saw it
persisted, and nothing ever consulted it. Retention stayed off. The page now
writes the key that is read, the duplicate is out of the seed list, and
migration 7d39 carries a value an operator actually set on the dead key over to
the live one before deleting it: their intent was "keep 30 days", and the fix
should not silently discard it. The page's copy also claimed logs "will be
automatically purged", which nothing does - purging is manual, from the site
settings page or `flask audit purge --apply`.

The Authentication page offered a full SAML configuration - metadata URL, entity
id, ACS URL, auto-create, admin group - and reported "Configured - SAML login
available" once two fields held text. There is no SAML anywhere in the codebase:
no login route, no ACS, no assertion handling. SAML IS planned, so the settings
stay and the page keeps them; what changes is that it now says plainly that
nothing consumes them yet, and the status line never claims availability.

The dangerous control there was "Allow Local Login". Turning it off did nothing
at all, so an administrator could believe they had closed local password login
while it stayed wide open. It is locked on until there is another way in, and
says why. CONFIG.md marks the whole block reserved rather than describing it as
working configuration.

Scope: product

shopdb/core/api/settings.py

@@ -1460,14 +1460,13 @@ def build_default_settings():
                            'Incoming Webhook), adaptivecard (Teams Workflow), '
                            'or json (generic {title,text})'
         },
-        # Audit log settings
-        {
-            'key': 'audit_retention_days',
-            'value': '90',
-            'valuetype': 'integer',
-            'category': 'audit',
-            'description': 'Number of days to retain audit logs (0 = keep forever)'
-        },
+        # Audit log retention lives at 'audit_retentiondays' (category site),
+        # defined above and read by shopdb/core/services/auditretention.py. A
+        # second key 'audit_retention_days' was seeded here with a different
+        # default and a different category, and the Audit & Logging page wrote
+        # to THAT one - so the page saved a value, read it back, and nothing
+        # ever consulted it. Removed rather than aliased: two keys for one
+        # setting is how this happened.
         # Authentication settings
         {
             'key': 'saml_enabled',