Expose max tool-call rounds per chat turn as admin-configurable #75

Context

docs/CONFIG_AUDIT_2026-09-04.md found that backend/app/agents/chat_agent.py:16 hardcodes MAX_TOOL_CALL_ROUNDS = 5 — capping how many tool-call round-trips (e.g. repeated search_knowledge_base calls) one chat turn can make before the agent is forced to answer. This is a classic operational ceiling (cost/latency vs. thoroughness tradeoff), in the same spirit as the already-DB-backed rate-limit capacity fields, but isn't itself configurable.

Scope

1. Add nullable SystemSettings column chat_max_tool_call_rounds (int, ge=1), env-default fallback.
2. New or extended resolver (likely a small addition to an existing resolver, or its own resolve_chat_agent_config if none fits — follow docs/BACKEND_SERVICES.md).
3. Wire into chat_agent.py, replacing the module constant.
4. Add to the Settings UI (likely a new small "Chat Agent" section, or folded into an existing one if a natural fit emerges during implementation) with a tooltip.
5. Audit-log on change.

Definition of Done

• chat_max_tool_call_rounds nullable column + Alembic migration.
• Resolver function, unit-tested.
• Wired into chat_agent.py.
• Settings UI field + tooltip.
• Audit logging on change.
• RBAC call-out per AGENTS.md TDD Discipline step 0: admin feature.
• Standard merge gate from AGENTS.md's Definition of Done applies in full.