Add check_container_health helper, wired into mattermost.sh as reference

A service's "Started" message after docker compose up -d doesn't mean
the app is actually working — it can still crash-loop (bad DB
password, missing required env var, etc.) with no visible sign until
someone separately runs docker ps -a much later, exactly what happened
repeatedly this session (mattermost, koha-db, homebox, vaultwarden,
filebrowser all showed a clean "Started" message while crash-looping).

check_container_health (lib/common.sh) waits briefly, checks the
container's actual status and restart count via docker inspect, and
prints recent logs automatically if it's not running or has already
restarted — instead of a misleading one-line success message.

Wired into mattermost.sh's own start step as the reference
implementation, guarded by declare -F so standalone runs (no
lib/common.sh sourced) degrade gracefully. Not retrofitted across
every other service in one pass — this establishes the shared helper
so other services can adopt it incrementally.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H4k6J1qXXyYxhGEgnJaMvn

services/mattermost.sh

@@ -742,9 +742,19 @@ MIGRATE_BODY
     local START=""
     prompt_yn "Start Mattermost now? (y/n):" "y" START
     if [ "$START" = "y" ] || [ "$START" = "Y" ]; then
-        docker compose up -d \
-            && log_success "Mattermost started" \
-            || log_warning "Start failed — check: docker compose logs"
+        if docker compose up -d; then
+            log_success "Mattermost started"
+            # Reference implementation of the shared health check — a
+            # "Started" message alone doesn't mean the app is actually up;
+            # it can still crash-loop (bad DB password, missing required
+            # env var, etc.) with no visible sign until someone separately
+            # runs `docker ps -a` much later. Mattermost's own first DB
+            # connection attempt can take a few seconds, hence the longer
+            # wait than check_container_health's 8s default.
+            declare -F check_container_health >/dev/null 2>&1 && check_container_health "$MM_CONTAINER" 12
+        else
+            log_warning "Start failed — check: docker compose logs"
+        fi
     fi
 
     echo ""