Add lockfile to prevent concurrent immich-sync runs

- Creates ~/.immich-sync.lock with PID on start
- Checks if existing lock's process is still running (stale lock detection)
- Logs clear message when skipping due to active lock
- Releases lock on exit via trap (handles success and failure)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ansible/roles/immich_sync/templates/immich-sync.sh.j2

@@ -18,6 +18,7 @@ DOCKER=/usr/local/bin/docker
 EXPORT_DIR="{{ immich_sync_export_dir }}"
 IMMICH_URL="{{ immich_sync_url }}"
 API_KEY_FILE="$HOME/.immich-api-key"
+LOCKFILE="$HOME/.immich-sync.lock"
 LOG_PREFIX="[immich-sync]"
 
 log() {
@@ -28,6 +29,43 @@ error() {
     echo "$LOG_PREFIX $(date '+%Y-%m-%d %H:%M:%S') ERROR: $*" >&2
 }
 
+# Lockfile management to prevent concurrent runs
+acquire_lock() {
+    if [[ -f "$LOCKFILE" ]]; then
+        local lock_pid lock_time
+        lock_pid=$(cat "$LOCKFILE" 2>/dev/null | head -1)
+        lock_time=$(stat -f %Sm -t '%Y-%m-%d %H:%M:%S' "$LOCKFILE" 2>/dev/null || echo "unknown")
+
+        # Check if the process is still running
+        if [[ -n "$lock_pid" ]] && kill -0 "$lock_pid" 2>/dev/null; then
+            error "Another sync is already running (PID: $lock_pid, started: $lock_time)"
+            error "Lockfile: $LOCKFILE"
+            error "If this is stale, remove the lockfile manually: rm $LOCKFILE"
+            exit 0  # Exit cleanly so LaunchAgent doesn't report failure
+        else
+            log "WARNING: Found stale lockfile from PID $lock_pid (process not running), removing it"
+            rm -f "$LOCKFILE"
+        fi
+    fi
+
+    # Create lockfile with our PID
+    echo $$ > "$LOCKFILE"
+    log "Acquired lock (PID: $$)"
+}
+
+release_lock() {
+    if [[ -f "$LOCKFILE" ]]; then
+        rm -f "$LOCKFILE"
+        log "Released lock"
+    fi
+}
+
+# Ensure lock is released on exit (success or failure)
+trap release_lock EXIT
+
+# Acquire lock before doing anything else
+acquire_lock
+
 # Check prerequisites
 if [[ ! -x "$MISE" ]]; then
     error "mise not found at $MISE"