feat(monitor): add API Key authentication for server heartbeat

- Add api_key field to MonitoredServer model
- Add migration to create api_key column with unique index
- Add /admin/servers/{id}/api-key endpoint for key generation
- Add /admin/servers/{id}/api-key DELETE endpoint for revocation
- Add /server/heartbeat-v2 endpoint with X-API-Key header authentication
- Add TelemetryPayload model with extended fields (load_avg, uptime_seconds)
- Add basic unit tests for API key functionality

app/main.py

@@ -215,6 +215,11 @@ def _migrate_schema():
                 "DEFAULT 'open'"
             ))
 
+        # --- monitored_servers.api_key for heartbeat v2 ---
+        if _has_table(db, "monitored_servers") and not _has_column(db, "monitored_servers", "api_key"):
+            db.execute(text("ALTER TABLE monitored_servers ADD COLUMN api_key VARCHAR(64) NULL"))
+            db.execute(text("CREATE UNIQUE INDEX idx_monitored_servers_api_key ON monitored_servers (api_key)"))
+
         db.commit()
     except Exception as e:
         db.rollback()