Switch deploy pipeline to Gitea polling; fix env-parsing and volume-isolation bugs

- Replace the bare-repo post-receive hook with deploy/poll-deploy.sh: Gitea
  and the Docker hosts are separate machines, so each box polls its branch
  via host crontab instead of needing an exposed webhook receiver.
- Add Blocksy theme + Blocksy Companion auto-install to deploy.sh (free
  tier; the paid Book Store starter site still needs a manual license step).
- Fix deploy.sh/backup.sh/restore.sh sourcing .env files as bash: a bcrypt
  hash's `$2a$14$...` shape breaks under `set -u`. Replaced with
  deploy/lib/env.sh, a literal (non-executing) KEY=VALUE reader.
- Fix docker compose itself mangling the same kind of value: both
  `environment: ${VAR}` and `env_file:` run values through Compose's
  interpolation, which silently blanks `$identifier`-shaped substrings.
  The staging basic-auth hash is now rendered directly into the Caddyfile
  by deploy.sh, bypassing Compose's variable system entirely.
- Fix dev/staging/production silently sharing one Compose project (and
  therefore one db_data volume) by pinning an explicit -p per environment.
- cron and wordpress now share one environment anchor so they can't drift
  apart again (cron was silently missing WORDPRESS_CONFIG_EXTRA before).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -2,6 +2,14 @@
 # values for that environment. The filled-in files are gitignored — never
 # commit them. Same code everywhere; only these values differ (see
 # design doc §01, Environments & deployment).
+#
+# CAVEAT for DB_PASSWORD, DB_ROOT_PASSWORD, and the API keys below: docker
+# compose passes these into containers via ${VAR} interpolation, which will
+# silently mangle a value containing `$` followed by a letter (it tries to
+# resolve it as another variable and blanks it out if unset). Avoid `$` in
+# these specific values, or double it ($$) if you must use one. This does
+# NOT apply to STAGING_BASIC_AUTH_HASH below — that one's wired through
+# env_file instead specifically so a bcrypt hash's `$` signs are safe.
 
 # --- Site ---
 # (WP_ENVIRONMENT_TYPE is NOT set here — it's hardcoded per environment in