fix(compose): pin CERTCTL_DATABASE_URL in demo overlay (cold-DB smoke fix #4)

Fourth latent bug surfaced by the Auditable Codebase Bundle's
cold-DB compose smoke. CI run on master tip d92a0c98 fails with:

  certctl-postgres | FATAL: password authentication failed for user
  "certctl" (SQLSTATE 28P01 — invalid_password)

after every other auth gate has been satisfied. The earlier closures
(4737fff DEMO_MODE_ACK, f1aecc6 migration 000043 idempotency,
ec72c69 bootstrap-token interpolation) all hold; this one is a
different interpolation gap.

Root cause: the base compose at deploy/docker-compose.yml:177 builds
the certctl-server's database URL via compose-level interpolation:

  CERTCTL_DATABASE_URL: ${CERTCTL_DATABASE_URL:-postgres://certctl:${POSTGRES_PASSWORD}@postgres:5432/certctl?sslmode=disable}

The inner ${POSTGRES_PASSWORD} reads the SHELL environment, not the
postgres service's environment: block. The demo overlay sets
POSTGRES_PASSWORD: certctl on the postgres service (which feeds
postgres's initdb only — that's why the database is seeded with
password 'certctl'), but never exports it as a compose-level shell
var. In a zero-env-var CI run the shell var is blank, so the
generated URL is:

  postgres://certctl:@postgres:5432/certctl?sslmode=disable
                    ^ empty password

while postgres rejects with SCRAM mismatch because its pg_authid
holds the hash of 'certctl'.

Pre-CI, this gap was masked because every developer running the
demo locally had POSTGRES_PASSWORD=certctl in their shell or
deploy/.env from earlier sessions; the cold-DB smoke is the first
zero-env-var consumer of this overlay.

Fix: pin CERTCTL_DATABASE_URL with the literal demo password in the
demo overlay's certctl-server environment block. The base compose's
${CERTCTL_DATABASE_URL:-...} default is overlay-overridable, so this
literal is overlay-scoped — production deploys that supply their
own CERTCTL_DATABASE_URL still win. The overlay was always claimed
self-sufficient by its docstring ('Supplies the change-me-...
placeholder values for POSTGRES_PASSWORD, CERTCTL_API_KEY,
CERTCTL_CONFIG_ENCRYPTION_KEY, and CERTCTL_AGENT_ID so the demo
runs without a deploy/.env file') — this commit makes the database
URL actually match that claim.

Same pattern as the ec72c69 BOOTSTRAP_TOKEN fix: when compose-level
interpolation reads from the shell, the overlay's environment:
block alone is not enough; the variable that references it must
also be pinned explicitly.

Verified:
  YAML parse clean (python3 yaml.safe_load).
  All 35 scripts/ci-guards/*.sh green, including
    complete-path-config-coverage.sh (CERTCTL_DATABASE_URL has a
    non-config consumer in deploy/), G-3-env-docs-drift,
    B2-compose-base-no-demo-env, S-1-hardcoded-source-counts.

deploy/docker-compose.demo.yml

@@ -76,6 +76,22 @@ services:
       # sentinels outside demo mode, but DEMO_MODE_ACK=true unlocks them.
       CERTCTL_CONFIG_ENCRYPTION_KEY: change-me-32-char-encryption-key
       CERTCTL_AUTH_SECRET: change-me-in-production
+      # Cold-DB smoke fix (2026-05-13): the base compose builds the
+      # database URL via compose-level `${POSTGRES_PASSWORD}` interpolation
+      # (deploy/docker-compose.yml line ~177), which reads the SHELL env —
+      # NOT the postgres service's `environment:` block above (that one
+      # feeds the postgres container's initdb only). In a zero-env-var
+      # CI run the shell var is blank, producing
+      # `postgres://certctl:@postgres:5432/...` and a SCRAM rejection
+      # against a database that initdb seeded with password `certctl`.
+      # Pinning the full URL here closes the gap: the demo overlay is
+      # now fully self-sufficient (matches the file's docstring claim)
+      # and the cold-DB smoke passes against a fresh GitHub-runner clone
+      # with no .env file or exported shell vars. Production deploys
+      # override CERTCTL_DATABASE_URL via the base compose's
+      # `${CERTCTL_DATABASE_URL:-...}` default, so this literal is
+      # overlay-scoped and never leaks into a production posture.
+      CERTCTL_DATABASE_URL: postgres://certctl:certctl@postgres:5432/certctl?sslmode=disable
       # 180-day simulated history seed applied at boot.
       CERTCTL_DEMO_SEED: "true"