feat(platform): T01 — standalone PostgreSQL HA chart scaffold

Lays out the S3 platform layer foundation for RAIL-PL-WP-0001 T01:

- .sops.yaml: age encryption policy (shared key, *.sops.yaml pattern)
- .gitignore: prevents accidental commit of decrypted values files
- Makefile: pg-deploy, pg-status, pg-pgpool-check, valkey-deploy,
  valkey-status, backup targets with KUBECONFIG/HELM wiring
- helm/postgresql-ha-values.yaml.template: annotated values schema
  with CHANGEME_ placeholders; includes pgpool-password fix from
  RAIL-BS-WP-0003; notes on single-node vs ThreePhoenix scaling
- docs/postgresql-ha.md: connection strings, DB creation, password
  rotation, pgpool-password critical note, HA failover test ref,
  ThreePhoenix scaling path

To complete T01: fill in CHANGEME_ values, encrypt with sops -e -i,
then run make pg-deploy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,7 @@
+# Decrypted helm values — never commit plaintext secrets
+helm/*.yaml
+!helm/*.sops.yaml
+!helm/*.yaml.template
+
+# Kubeconfig
+*.kubeconfig