Fix DB/admin password regeneration on rerun in 5 services

Same bug class just fixed in mattermost.sh: immich, joplin, koha,
mail-archiver, and nextcloud all generated a fresh random
DB/admin password on every single run with no check for an
existing one. Each backs its database with a persistent volume, so
Postgres/MariaDB keeps the password from its first init while the
freshly overwritten .env (or config-main.env for koha) no longer
matches it — any rerun would have locked the app out of its own
database. koha, mail-archiver, and nextcloud also regenerated an
app-level admin login password the same way.

Found by cross-referencing every service with a DB password against
which ones actually guard reuse on rerun (only traccar.sh did,
already correctly) rather than waiting to be told about each one
individually.

Fix mirrors traccar.sh's existing pattern: read the password back out
of the existing .env/config file if present, only generate fresh when
there's genuinely nothing there yet.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NQkdAn3iG5A4WoqU9FHMaN

services/joplin.sh

@@ -197,8 +197,12 @@ install_joplin() {
     ensure_docker_dir_ownership "$JOPLIN_DIR"
     cd "$JOPLIN_DIR" || return 1
 
-    local DB_PASS
-    DB_PASS="$(generate_password 32)"
+    # Reused across reruns if already set — the Postgres volume keeps the
+    # password from its first init, so a fresh random one on every rerun
+    # would lock Joplin out of its own database.
+    local DB_PASS=""
+    [ -f ".env" ] && DB_PASS="$(grep '^POSTGRES_PASSWORD=' .env | cut -d= -f2-)"
+    [ -n "$DB_PASS" ] || DB_PASS="$(generate_password 32)"
     local BASE_URL="https://joplin.${SITE_DOMAIN}"
 
     # Mirrors configure_caddy_for_service's own mode resolution (lib/common.sh):