feat: guard against a dead Instagram session, show progress, fix missing pings

- check-session.py: preflight in gdl-cron.sh. No unexpired sessionid in the
  Chrome profile gdl reads -> abort before any request, ping /76 (red check).
- stories-streak.py: postflight. Every fetched stories source empty for 3
  consecutive days -> exit 77, for a session Instagram stopped honouring while
  the cookie is still present. The 2026-10-07 session death went unnoticed for
  a week because every run still exited 0.
- gdl-sync.py: [n/N] source headers, a 60s heartbeat with new-file counts, and
  per-source timing, so a healthy paced run no longer looks hung.
- gdl-sync@.service now runs run-%i.sh instead of gdl-cron.sh: HC_URL is set in
  the wrappers, so scheduled runs had never pinged Healthchecks.
- Tests for both guards (offline) and the progress helpers.

scripts/systemd/gdl-sync@.service

@@ -8,7 +8,7 @@ Wants=network-online.target
 
 [Service]
 Type=oneshot
-ExecStart=%h/gdl/gdl-cron.sh %i
+ExecStart=%h/gdl/run-%i.sh
 # A sync has no deadline and the pacing is deliberately slow; a full sweep can
 # run for hours. Never let systemd kill one midway -- a half-published run is
 # the one state the publish step is designed to avoid.