Cap persisted log messages at 5000 chars so long langgraph runs no longer accumulate 10 KB ``ResearchLog`` rows. Mirrors PR #4004's frontend truncation discipline, now applied to the database sink. Also promotes the duplicated ``/history/logs`` pagination-cap literals into a single shared constant (``HISTORY_LOGS_HARD_CAP`` / ``HISTORY_LOGS_DEFAULT_LIMIT``, exposed to the log panel as ``window.LDR_LOG_LIMITS``) so the route clamp and the panel's DOM cap no longer drift. Full diagnostics remain available in stderr/file sinks.
