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/models/setting.py

@@ -0,0 +1,16 @@
+"""Admin-editable runtime settings (registration_open, flag_threshold, etc.)."""
+from datetime import datetime
+from sqlalchemy import JSON
+from app.extensions import db
+
+
+class Setting(db.Model):
+    __tablename__ = "settings"
+
+    key = db.Column(db.String(80), primary_key=True)
+    value = db.Column(JSON, nullable=True)
+    updated_at = db.Column(db.DateTime, default=datetime.utcnow,
+                           onupdate=datetime.utcnow, nullable=False)
+
+    def __repr__(self):
+        return f"<Setting {self.key}>"