06/16 Phase 6 foundation: admin blueprint, audit log, user management

Adds the admin backend foundation: AuditLog + Setting models/migration,
an admin-only dashboard with KPI cards, and user management (search,
ban/suspend/activate, tier override, trust adjust) with audit logging
on every write action. Smoke suite extended to 64 checks.

app/services/settings.py

@@ -0,0 +1,18 @@
+"""Admin-editable runtime settings, stored as key/JSON-value rows."""
+from app.extensions import db
+from app.models.setting import Setting
+
+
+def get_setting(key: str, default=None):
+    row = db.session.get(Setting, key)
+    return row.value if row is not None else default
+
+
+def set_setting(key: str, value):
+    row = db.session.get(Setting, key)
+    if row is None:
+        row = Setting(key=key, value=value)
+        db.session.add(row)
+    else:
+        row.value = value
+    return row