Enable zstd WAL compression in PostGIS config

Reduces WAL volume (and thus disk I/O and archive/replication traffic)
by compressing full-page images with zstd.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: aapeliv

app/postgis/postgresql.conf

@@ -18,6 +18,7 @@ dynamic_shared_memory_type = posix
 wal_buffers = 64MB
 max_wal_size = 2GB
 min_wal_size = 80MB
+wal_compression = zstd
 
 # Planner (SSD)
 random_page_cost = 1.1