[RESILIENCE] Automatic WAL archiving and truncation — prevent WAL infinite growth

[ElioNeto]

Description

After per-CF WAL (#157), each column family has its own WAL. If a CF is written heavily but never flushed, its WAL grows unbounded, eventually filling the disk.

Implementation

1. Add WAL size limit per CF (configurable, default: 512MB)
2. When a WAL exceeds the limit:
   • Force flush the CF's memtable
   • Archive the WAL segment to <db>/wal-archive/{cf}-{seq}.wal
   • Create a fresh WAL for the CF
3. Archive retention policy:
   • Keep last N archive segments
   • Auto-delete archives older than T days
4. WAL archive serves as additional crash recovery layer

Configuration

resilience:
  wal:
    max_size_per_cf_mb: 512
    archive_retention_count: 10
    archive_retention_days: 7

Labels

[ElioNeto]

Implemented in commit 0871d91.