Summary

Implements core audit log sequencer (PR # 2) with guaranteed gap-free sequence numbers (1, 2, 3...) for reliable log aggregation and integrity monitoring.

Key Features

• Gap-free sequencing: Single-writer pattern ensures no gaps under concurrent load (verified with 10k events from 100 concurrent clients)
• Backpressure handling: Non-blocking queue submission with error propagation when overloaded
• Service layer: Clean separation of domain logic, data layer, and orchestration
• UTF-8 safe validation: Payload size limits with character boundary preservation
• Graceful shutdown: Context-based timeout coordination matching server patterns

Implementation Details

• Config management with production defaults and comprehensive validation
• Store method EnsureUniqueBytebaseID with collision retry (up to 5 attempts)
• Database migration: UNIQUE constraint on (bytebase_id, sequence_number)
• Error handling: common.Error with proper codes (Invalid, Internal, SizeExceeded)
• Test coverage: 12 tests including race detector and concurrent load verification

Files Changed

• backend/common/audit/: config.go, logger.go, service.go, validation.go, instance.go (refactored)
• backend/store/audit_log.go: Added EnsureUniqueBytebaseID method
• backend/migrator/migration/3.9/0011##audit_log_unique_bytebase_id.sql: UNIQUE index
• Tests: 575 LOC comprehensive test suite

Dependencies

Requires PR #17897 (proto fields + Bytebase ID generation)