Part 1 of 6 for Audit Logs to Stdout feature (v3.15)

Adds foundational proto fields and store methods for gap-free audit log sequences, enabling stdout streaming with deduplication and gap detection in downstream log aggregators.

Changes:

• Add instance_id and sequence_number fields to AuditLog proto
• Implement instance ID generation with collision detection (audit/instance.go)
• Add store methods: GetMaxAuditSequence(), CheckInstanceIDExists()
• Integration test verifying legacy logs (NULL sequences) coexist with new logs

Key Details:

• New audit logs will get sequences (1, 2, 3...) - assigned in PR # 2
• Old logs remain queryable with NULL sequences (100% backward compatible)
• No schema migrations or backfill required
• Instance ID persists across restarts (hostname/env-based)

Testing:

• Unit tests pass with race detector
• golangci-lint clean
• Integration test validates legacy/new log coexistence

Next: PR # 2 will implement Logger with sequencer goroutine for stdout streaming