fix(deploy,examples,env): close U-1 trap end-to-end across Helm, examples, and root env

Follow-up to 78dcc9e (U-1 docker-compose fix) — closes the remaining adjacent
code paths that share the postgres-first-boot-password-binding root cause but
were scoped out of the original commit.

The runtime diagnostic in internal/repository/postgres/db.go::wrapPingError
(landed in 67f352d) already covers every NewDB call site, so Helm operators
and example users hit the SQLSTATE 28P01 guidance for free at startup. What
was missing: deployment-shape-specific remediation guidance (kubectl vs
docker-compose), the hardcoded password in the *root* .env.example, and
shared ops notes for the 5 examples/ compose files. This commit closes all
three.

Files changed:

- .env.example (root) — line 16 had `postgres://certctl:certctl@...` with
  the password hardcoded literally instead of interpolating POSTGRES_PASSWORD.
  Edit if a user copied this file as their .env (binary-direct deployment,
  not docker-compose) and rotated POSTGRES_PASSWORD on line 10, the URL on
  line 16 still carried 'certctl' — silent two-line drift. Replaced 'certctl'
  with the same default that line 10 carries ('change-me-in-production') and
  added an explanatory comment block describing the docker-compose
  override semantics, when this URL matters (binary-direct), and the
  cross-reference to the U-1 wrapPingError diagnostic. Also fixed an
  adjacent bug: line 31 CERTCTL_SERVER_URL was `http://localhost:8443`,
  which agents reject at startup since v2.2 (HTTPS-everywhere milestone made
  the control plane HTTPS-only with TLS 1.3 pinned). Updated to https://
  with a comment pointing operators at the bootstrap CA bundle.

- deploy/helm/certctl/values.yaml — postgresql.auth.password field had a
  one-line 'REQUIRED' comment. Expanded into a full WARNING block (~25
  lines) explaining the PVC retention semantics, the failure symptom,
  and both kubectl-flavored remediation paths: non-destructive
  (`kubectl exec ... ALTER ROLE`) preferred for environments with data,
  and destructive (`helm uninstall + kubectl delete pvc`) for dev/demo.
  Cross-references the wrapPingError runtime diagnostic.

- deploy/helm/certctl/README.md (new, ~115 lines) — chart-level operational
  guide. Covers quick install, both remediation paths with concrete
  kubectl commands, why-we-don't-fix-this-in-the-chart explanation,
  cross-references to the docker-compose docs, server API key rotation
  (the easy case — comma-separated key list), TLS provisioning shapes,
  embedded-vs-external postgres, and uninstall semantics with the PVC
  retention gotcha called out.

- examples/README.md (new, ~55 lines) — shared operational notes for the
  5 example deployments. Covers the postgres password rotation trap with
  example-flavored remediation paths (`docker compose -f examples/<x>/...`),
  the TLS warning, and teardown semantics. Replaces what would otherwise
  be 5x duplication across per-example READMEs.

- examples/{acme-nginx,acme-wildcard-dns01,multi-issuer,private-ca-traefik,
  step-ca-haproxy}/*.md — one-line cross-reference at the top of each
  example's primary doc, pointing at examples/README.md for the shared
  ops notes. Avoids 5x duplication of the same warning text while still
  surfacing the link in every operator's first-touch surface.

Verification:

- go build ./... — clean
- go vet ./... — clean
- go test -short ./internal/repository/postgres/ — 4/4 wrapPingError tests
  still passing (no production-code touch in this commit)
- helm lint deploy/helm/certctl/ — clean (1 INFO about chart icon, pre-existing)
- helm template smoke test — renders without error
- python3 yaml.safe_load on values.yaml — parses

Refs: coverage-gap-audit-2026-04-24-v5/unified-audit.md
      §2 P1 cluster, cat-u-quickstart_postgres_password_volume_trap
      Closes the three deliberate scope-outs from 78dcc9e (Helm,
      root .env.example, examples/) end-to-end.

      Adjacent bugs caught while in scope:
      - root .env.example:16 hardcoded password not matching line 10
      - root .env.example:31 http:// URL incompatible with HTTPS-only v2.2

deploy/helm/certctl/values.yaml

@@ -260,7 +260,30 @@ postgresql:
   auth:
     database: certctl
     username: certctl
-    password: ""  # REQUIRED - set via --set or values override
+    # REQUIRED — set via `--set postgresql.auth.password=<value>` or values override.
+    #
+    # WARNING (U-1): rotating this value after first deploy does NOT change the
+    # database password. The `postgres:16-alpine` image runs `initdb` only when
+    # /var/lib/postgresql/data is empty, so POSTGRES_PASSWORD is written into
+    # pg_authid exactly once — on the first boot of the StatefulSet's PVC.
+    # Subsequent rollouts pick up the new env value in the postgres container
+    # but the certctl-server container's CERTCTL_DATABASE_URL also picks up
+    # the new value, while pg_authid still expects the old one — leading to
+    # `pq: password authentication failed for user "certctl"` (SQLSTATE 28P01).
+    #
+    # The certctl-server emits guidance via internal/repository/postgres/db.go::
+    # wrapPingError when it sees SQLSTATE 28P01 at startup. To resolve in a
+    # Helm deployment:
+    #   - Non-destructive (preferred for environments with data):
+    #       kubectl exec -it <release>-postgres-0 -- \
+    #         psql -U certctl -c "ALTER ROLE certctl PASSWORD '<new>';"
+    #     then update the secret/values to match and let the certctl-server
+    #     pod restart against the matching credential.
+    #   - Destructive (DESTROYS DATA — only acceptable on dev/demo PVCs):
+    #       helm uninstall <release> && \
+    #       kubectl delete pvc -l app.kubernetes.io/name=certctl,app.kubernetes.io/component=postgres && \
+    #       helm install <release> ...  # PVC re-creates empty, initdb seeds new password
+    password: ""
 
   # Storage configuration
   storage: