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>

.sops.yaml

@@ -0,0 +1,12 @@
+# SOPS encryption policy for railiance-platform
+# Encrypts any file matching *.sops.yaml using the shared age key.
+# Decrypt: sops -d helm/postgresql-ha-values.sops.yaml
+# Use with helm: helm upgrade postgresql-ha bitnami/postgresql-ha \
+#   -n platform -f <(sops -d helm/postgresql-ha-values.sops.yaml)
+# Encrypt: sops -e -i helm/postgresql-ha-values.sops.yaml
+
+creation_rules:
+  - path_regex: \.sops\.yaml$
+    key_groups:
+      - age:
+          - age1aq8twfd78wvpra0had8cezcnj96tj4q0068edrz5jez8d6xwmflqdepsh4