Add database selection best practice: SQLite is not a production database

Hard rule: any service with a FQDN, multiple consumers, or concurrent
access MUST use PostgreSQL from day one. Documents the cost of "we'll
migrate later" based on the Gitea SQLite→PostgreSQL migration that cost
nearly a full day of productivity.

Extracted from cluster-bootstrap gitea-scaling session (2026-03-28).

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

BESTPRACTICES.md

@@ -25,5 +25,7 @@ Generalised best practices extracted from real project work. Each topic file is
 - [Test-Driven Development](best-practices/test-driven-development.md) — Edge case discovery, property-based testing, mutation testing, AI agent testing patterns, test architecture
 - [Networking & Infrastructure](best-practices/networking.md) — nftables safety, systemd socket activation, Docker forwarding, TLS SNI vs Host header, wildcard certs
 - [Docker UID Matching](best-practices/docker-uid-matching.md) — UID wrapper entrypoint for mounted volumes, gosu pattern, when to use vs K8s securityContext
+- [Database Selection](best-practices/database-selection.md) — SQLite is not a production database; always use PostgreSQL for services with FQDNs, multiple consumers, or concurrent access
+- [Docker](best-practices/docker.md) — gosu PID 1, GIT_SSH_COMMAND scope, slim image health checks, buildx local images, default users, TTY flags, UID resolution
 - [Octopus Process Templates](best-practices/octopus-process-templates.md) — OCL syntax, step template references, channel scoping, parameters, versioning, Platform Hub patterns
 - [Platform Hub Knowledge Base](PLATFORMHUB.md) — Comprehensive guide: architecture, OCL syntax, template patterns, gotchas, API reference