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/stories-streak.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+"""
+Detect a silently dead Instagram session from a stories run's output.
+
+A dead session (killed server-side while the sessionid is still in the cookie jar, so
+check-session.py passes) shows up as every fetched stories source returning
+"No results" -- the daily run still exits 0. From 2026-10-07 all six profiles came back
+empty every day and nothing alerted. One all-empty day is normal (nobody posted a story);
+several in a row across ALL sources is not.
+
+Usage: stories-streak.py <this run's output file> [state file]
+Counts at most one day per calendar date (manual re-runs don't inflate it). Prints a message
+and exits 1 once the streak reaches GDL_EMPTY_STREAK_FAIL days (default 3); exits 0 otherwise.
+Runs that fetched nothing (all skipped / no sources) leave the streak untouched.
+"""
+import datetime as dt
+import os
+import re
+import sys
+from pathlib import Path
+
+THRESHOLD = int(os.environ.get("GDL_EMPTY_STREAK_FAIL", "3"))
+
+
+def main() -> int:
+    out = Path(sys.argv[1]).read_text(errors="replace")
+    state = Path(sys.argv[2] if len(sys.argv) > 2
+                 else Path.home() / "gdl" / "stories-empty-streak")
+    m = re.search(r"sources\s*:\s*(\d+) to sync", out)
+    to_sync = int(m.group(1)) if m else 0
+    if to_sync == 0:
+        return 0                                    # nothing fetched: no evidence either way
+    empty = len(re.findall(r"No results for \S*/stories/", out))
+    today = dt.date.today().isoformat()
+    last_date, streak = "", 0
+    try:
+        last_date, s = state.read_text().split()
+        streak = int(s)
+    except (OSError, ValueError):
+        pass
+    if empty >= to_sync:                            # every fetched source came back empty
+        if last_date != today:
+            streak += 1
+    else:
+        streak = 0
+    state.write_text(f"{today} {streak}\n")
+    if streak >= THRESHOLD:
+        print(f"SESSION LIKELY DEAD: stories came back empty for ALL {to_sync} profile(s) on "
+              f"{streak} consecutive day(s). The cookie is present but Instagram is probably "
+              "no longer honouring it -- log in again in the Chrome window (reload the tab to "
+              "check) and re-run.")
+        return 1
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())