docs(infra): add PostgreSQL tuning and systemd service documentation

- POSTGRESQL_TUNING.md: documents applied config (8GB shared_buffers,
  64MB work_mem, 8GB max_wal_size, SSD params)
- SYSTEMD_SERVICES.md: lists all production systemd services
- systemd/: versioned copies of all .service and .timer files
- .gitignore: ignore package-lock.json and backups/

docs/POSTGRESQL_TUNING.md

@@ -0,0 +1,40 @@
+# PostgreSQL Tuning — Nexus Autoparts
+
+**Server:** 48 GB RAM, 8 cores, SSD (QEMU)  
+**Applied:** 2026-04-26  
+**Requires restart:** Yes (done)
+
+## Configuration Changes
+
+File: `/etc/postgresql/17/main/postgresql.conf`
+
+| Parameter | Before | After | Rationale |
+|-----------|--------|-------|-----------|
+| `shared_buffers` | 128 MB | **8 GB** | ~25% of RAM for PostgreSQL buffer cache |
+| `work_mem` | 4 MB | **64 MB** | Larger sorts/joins without disk spilling |
+| `maintenance_work_mem` | 64 MB | **1 GB** | Faster VACUUM, CREATE INDEX, ALTER |
+| `effective_cache_size` | 4 GB | **36 GB** | Planner knows OS cache is large |
+| `max_wal_size` | 1 GB | **8 GB** | Fewer checkpoints under heavy write load |
+| `checkpoint_completion_target` | 0.5 | **0.9** | Spread checkpoint I/O over more time |
+| `wal_buffers` | - | **16 MB** | WAL buffer sizing |
+| `random_page_cost` | 4.0 | **1.1** | SSD-appropriate random read cost |
+| `effective_io_concurrency` | 1 | **200** | SSD can handle many concurrent requests |
+| `max_connections` | 100 | **200** | Headroom for Celery, Quart, Dashboard, PgBouncer |
+
+## Verification
+
+```bash
+sudo -u postgres psql -d nexus_autoparts -c "SHOW shared_buffers;"
+```
+
+## Backup
+
+A backup of the previous config is stored at:
+`/etc/postgresql/17/main/postgresql.conf.backup.<timestamp>`
+
+## pg_hba Adjustment for Monitoring
+
+Added Docker network access for postgres-exporter:
+```
+host    nexus_autoparts    postgres    172.17.0.0/16    trust
+```