feat(db): add indexes on impersonation tables (GitHub #95)

Add three indexes to prevent full table scans as session volume grows:
- impersonation_sessions(staff_id, status) for active-session lookup
- impersonation_sessions(client_id) for existing-session check
- impersonation_audit_logs(session_id) for audit log lookup by session

Migration 0011 applied and verified on dev database.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/db/migrations/meta/_journal.json

@@ -78,6 +78,13 @@
       "when": 1742500800000,
       "tag": "0010_impersonation_sessions",
       "breakpoints": true
+    },
+    {
+      "idx": 11,
+      "version": "7",
+      "when": 1742587200000,
+      "tag": "0011_impersonation_indexes",
+      "breakpoints": true
     }
   ]
 }