fix(agent): attribute rebalance logs per user/position, not first user

[bakarezainab]

Problem: logAgentAction queried db.user.findMany({ take: 1 }) and pinned every agent log row to an arbitrary first user, making auditing and per-user analytics unreliable.

Changes:

• schema.prisma: make AgentLog.userId nullable (system-level actions produce rows with userId=null, distinguishable from user-level ones); add positionId field; add composite index [userId, createdAt] for hot query paths
• prisma/migrations/20260617000000_fix_agent_log_attribution: SQL migration altering agent_logs accordingly
• router.ts: remove first-user lookup from logAgentAction; accept optional userId + positionId params; in triggerRebalance, fetch all affected positions and write one log row per (userId, positionId) pair; system-level REBALANCE/ANALYZE calls leave userId=null
• tests/unit/agent/logAgentAction.test.ts: 9 unit tests covering system-level (null userId), user-level attribution, multiple-user isolation, and error-handling
• tests/integration/agent/rebalance.integration.test.ts: 6 integration tests verifying per-user log creation for each impacted position, no arbitrary-user attribution, deduplication guard, and null-userId system-level fallback

All 50 tests pass (5 suites), tsc --noEmit clean

Closes #166