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/audit.py

@@ -0,0 +1,14 @@
+"""Append-only audit trail writer for admin actions."""
+from app.extensions import db
+from app.models.audit import AuditLog
+
+
+def log_action(actor, action: str, target_type: str, target_id, meta: dict | None = None):
+    """Append an AuditLog row. Caller commits."""
+    db.session.add(AuditLog(
+        actor_id=actor.id if actor else None,
+        action=action,
+        target_type=target_type,
+        target_id=target_id,
+        meta=meta,
+    ))