feat(monitor): store nginx telemetry for generic clients

- accept nginx installation status and sites-enabled list
- persist nginx fields in server state
- expose nginx data in monitor overview/admin views
- auto-migrate new server_states columns on startup

app/main.py

@@ -224,6 +224,12 @@ def _migrate_schema():
         if _has_table(db, "server_states") and not _has_column(db, "server_states", "plugin_version"):
             db.execute(text("ALTER TABLE server_states ADD COLUMN plugin_version VARCHAR(64) NULL"))
 
+        # --- server_states nginx telemetry for generic monitor client ---
+        if _has_table(db, "server_states") and not _has_column(db, "server_states", "nginx_installed"):
+            db.execute(text("ALTER TABLE server_states ADD COLUMN nginx_installed BOOLEAN NULL"))
+        if _has_table(db, "server_states") and not _has_column(db, "server_states", "nginx_sites_json"):
+            db.execute(text("ALTER TABLE server_states ADD COLUMN nginx_sites_json TEXT NULL"))
+
         db.commit()
     except Exception as e:
         db.rollback()