feat(infra): add pgBackRest for PostgreSQL Point-in-Time Recovery

Replace simple pg_dumpall with pgBackRest PITR backup system.
This enables recovery to any second, not just the last daily dump.

Configuration:
- docker/postgres/postgresql.conf: WAL archiving + performance tuning
  (shared_buffers=512MB, effective_cache_size=2GB for 16GB Mac Mini)
- docker/postgres/pgbackrest.conf: stanza config + retention policy

Docker (docker-compose.macmini.yml):
- postgres: mount custom config, enable WAL archiving
- postgres-backup: new pgBackRest container
  - Storage: /Volumes/ManaData/backups/pgbackrest
  - Retention: 4 full + 14 differential (~4 weeks)
  - Compression: Zstandard (zst)

Backup Schedule:
- 03:00 daily: Full backup
- Every 6h: Differential (changes since last full)
- Every hour: Incremental (changes since last backup)
- Continuous: WAL archiving (every 60s)

Documentation (docs/POSTGRES_BACKUP.md):
- Complete restore procedures (full, PITR, single DB)
- First-time setup instructions
- Monitoring and alerting integration

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/postgres/pgbackrest.conf

@@ -0,0 +1,22 @@
+[global]
+# Repository location (inside pgbackrest container, mapped to host SSD)
+repo1-path=/var/lib/pgbackrest
+repo1-retention-full=4
+repo1-retention-diff=14
+
+# Compression
+compress-type=zst
+compress-level=3
+
+# Parallelism
+process-max=2
+
+# Logging
+log-level-console=info
+log-level-file=detail
+
+[mana]
+pg1-path=/var/lib/postgresql/data
+pg1-host=postgres
+pg1-port=5432
+pg1-user=postgres

docker/postgres/postgresql.conf

@@ -0,0 +1,23 @@
+# PostgreSQL Configuration for WAL Archiving + pgBackRest
+# Append to default postgresql.conf
+
+# WAL Archiving (required for Point-in-Time Recovery)
+wal_level = replica
+archive_mode = on
+archive_command = 'pgbackrest --stanza=mana archive-push %p'
+archive_timeout = 60
+
+# WAL Settings (tuned for 16GB Mac Mini)
+max_wal_senders = 3
+max_wal_size = 1GB
+min_wal_size = 80MB
+
+# Performance (tuned for 16GB RAM, shared with other services)
+shared_buffers = 512MB
+effective_cache_size = 2GB
+work_mem = 16MB
+maintenance_work_mem = 128MB
+
+# Logging
+log_min_duration_statement = 1000
+log_checkpoints = on