Implement audit archiver service

[maxmalkin]

Summary

• Implement the audit archiver one-shot pipeline for audit log partition lifecycle management
• Create future partitions (7 days in advance), archive expired partitions to Parquet in S3-compatible cold storage, and drop archived partitions from PostgreSQL
• Add S3 and local filesystem cold storage backends with Hive-style key format
• Add PostgreSQL advisory lock leader election for single-instance guarantee
• Add Arrow schema (18 fields) and ZSTD-compressed Parquet writer
• Add MinIO to docker-compose for local S3-compatible development
• 18 unit tests covering partition logic, Parquet roundtrip, storage keys, config defaults, and local FS operations