docs: capture latency optimizations + new caching invariants

Shipping commit 88fb175 changed the trace shape and added a new caching
layer with required invalidation rules. Updating the operator-facing
docs so they match the running system.

ch08 (database):
- DB_HOST is the -pooler Neon endpoint, not direct compute
- Connection pool: MaxIdleConns 20 (was 10), MaxLifetime 30m (was 10m),
  MaxIdleTime 0 (never close idle)
- New \"Pool warm-up at boot\" section documenting the 20-parallel-ping
  warm-up in database.Connect
- Replaced the \"Neon regions\" section: explicit RTT numbers, the
  optimization stack that minimizes round-trips, when this still matters

ch15 (observability):
- Replaced the 2,473ms/5-span sample trace with the new 229ms/2-span
  post-optimization trace; kept the old one underneath for diff context

ch16 (failure modes):
- Added: stale residence-IDs cache (data freshness bug + recovery)
- Added: Redis at maxmemory limit (verify allkeys-lru policy)
- Added: Neon pooler unreachable but direct endpoint up — emergency
  switchover procedure

ch17 (runbook):
- §23 Invalidate residence-IDs cache for a user (DEL key + grep for
  missing invalidation in new code)
- §24 Verify DB pool warm-up is working (log pattern + impact test)
- §25 Switch DB host between pooler and direct endpoints

observability-plan.md status flipped from \"plan only\" to shipped
with the latency-cut summary.

README links to the new ch08 latency section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -378,6 +378,7 @@ Quick links:
 - **Deploy process** — [docs/deployment/14-deployment-process.md](./docs/deployment/14-deployment-process.md) — `bash deploy-k3s/scripts/03-deploy.sh` builds → pushes → rolls out
 - **Observability** — [docs/deployment/15-observability.md](./docs/deployment/15-observability.md) — VictoriaMetrics + Jaeger + Grafana on `obs.88oakapps.com`
 - **Observability plan** — [docs/observability-plan.md](./docs/observability-plan.md) — design doc and rollout phases
+- **Database / pool tuning** — [docs/deployment/08-database.md](./docs/deployment/08-database.md) — Neon pooler endpoint, GORM pool, warm-up, RTT budget
 - **Failure modes** — [docs/deployment/16-failure-modes.md](./docs/deployment/16-failure-modes.md) — what happens when X dies
 - **Swarm postmortem** — [docs/deployment/19-postmortem-swarm.md](./docs/deployment/19-postmortem-swarm.md) — why we migrated