fix: lower default max_slot_wal_keep_size from 4096MB to 512MB

The 4096MB default exceeds the total disk size of free tier projects
(2GB), risking disk exhaustion from a lagging or disconnected slot.
512MB fits safely within free tier disk overhead while still providing
a reasonable buffer for logical replication slots (e.g. Realtime, CDC).
WAL archiving to S3 means local retention is not needed for recovery.

Author: Crispy1975

ansible/files/postgresql_config/postgresql.conf.j2

@@ -299,7 +299,7 @@ max_wal_senders = 10		# max number of walsender processes
 max_replication_slots = 5	# max number of replication slots
 				# (change requires restart)
 #wal_keep_size = 0		# in megabytes; 0 disables
-max_slot_wal_keep_size = 4096   # in megabytes; -1 disables
+max_slot_wal_keep_size = 512    # in megabytes; -1 disables
 #wal_sender_timeout = 60s	# in milliseconds; 0 disables
 #track_commit_timestamp = off	# collect timestamp of transaction commit
 				# (change requires restart)