stacks: add .env + .env.secrets.ctmpl for productivity stacks

Adds real Vault Agent secret templates and non-secret env vars for
all 9 productivity stacks. Sanitized several real leaks: a real
residential public IP (ghost's UMAMI_IGNORE_IP), real personal
domains/email used across joplin/n8n/ghost/paperless-ngx, and a
real embedded OAuth client ID in paperless-ngx's private OIDC config
(dropped entirely - the public compose file doesn't wire up OIDC at
all, so none of that config was needed).

Also dropped n8n secrets referencing the out-of-scope Content
Pipeline system (nocodb-content, Postiz), and did not carry over a
real Redis password found sitting in plaintext in nextcloud's
private .env (unused by the public compose anyway).

Every var cross-checked against what its actual public compose file
references before writing.

stacks/productivity/nextcloud/.env

@@ -0,0 +1,6 @@
+# mysql credentials
+# MYSQL_ROOT_PASSWORD, MYSQL_PASSWORD — see .env.secrets (Vault)
+MYSQL_DATABASE=nextcloud
+MYSQL_USER=nextcloud
+
+TZ=Etc/UTC