feat: empty CRON_SCHEDULE keeps container alive for manual docker exec

Three container lifetime modes via CRON_SCHEDULE:
  unset          — run once on startup, exit
  empty string   — sleep infinity; use docker exec -it winnow winnow
  cron expression — run on startup, then on schedule

This replaces the need for a separate MANUAL_MODE env var. The empty
string is a natural "I want the container alive but unscheduled" signal.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

compose.yml

@@ -48,10 +48,14 @@ services:
       # - RESET_PERSON=John           # Clear uploaded+rejected history for one person
 
       # ── Scheduling ────────────────────────────────────────────────────────
-      # Cron expression (unset = run once and exit)
-      # - CRON_SCHEDULE=0 3 * * 0    # Every Sunday at 3 AM
-      # - CRON_SCHEDULE=0 3 1 * *    # First of every month
-      # - CRON_SCHEDULE=*/30 * * * * # Every 30 minutes
+      # CRON_SCHEDULE controls container lifetime:
+      #   unset          — run once on startup, then exit
+      #   empty string   — stay alive, run nothing; trigger manually with:
+      #                    docker exec -it winnow winnow
+      #   cron expression — run on startup, then on schedule
+      # - CRON_SCHEDULE=                # Manual mode (keep alive, no auto-run)
+      # - CRON_SCHEDULE=0 3 * * 0      # Every Sunday at 3 AM
+      # - CRON_SCHEDULE=0 3 1 * *      # First of every month
     volumes:
       # Replace with absolute paths on your host, e.g. /opt/winnow/models
       - /path/to/winnow/models:/models