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/test_gdl_sync.py

@@ -343,5 +343,43 @@ class PostUrlsFile(unittest.TestCase):
             ])
 
 
+class ProgressOutput(unittest.TestCase):
+    """The heartbeat that stops a healthy paced run (12-20s between requests, no per-file
+    output) from looking hung."""
+
+    def test_elapsed_formatting(self):
+        self.assertEqual(gdl._fmt_elapsed(5), "5s")
+        self.assertEqual(gdl._fmt_elapsed(125), "2m05s")
+        self.assertEqual(gdl._fmt_elapsed(7300), "2h01m")
+
+    def test_count_new_counts_media_written_since_and_ignores_sidecars(self):
+        with tempfile.TemporaryDirectory() as d:
+            root = Path(d)
+            (root / "sub").mkdir()
+            old = root / "old.jpg"
+            old.write_text("x")
+            import os
+            os.utime(old, (1, 1))                              # long before `since`
+            (root / "sub" / "new.jpg").write_text("x")
+            (root / "sub" / "new.mp4").write_text("x")
+            for sidecar in ("new.txt", "new.json", "half.jpg.part"):
+                (root / "sub" / sidecar).write_text("x")
+            n, newest = gdl._count_new(root, since=1000)
+            self.assertEqual(n, 2)
+            self.assertIn(newest, ("new.jpg", "new.mp4"))
+
+    def test_heartbeat_prints_while_running_and_stops_cleanly(self):
+        with tempfile.TemporaryDirectory() as d:
+            with mock.patch("builtins.print") as pr:
+                with gdl.Heartbeat("fetching", watch=Path(d), every=0.05) as hb:
+                    (Path(d) / "a.jpg").write_text("x")
+                    import time
+                    time.sleep(0.2)
+            lines = [str(c.args[0]) for c in pr.call_args_list]
+            self.assertTrue(any("fetching" in s and "new file(s) so far" in s for s in lines))
+            self.assertFalse(hb._thread.is_alive())
+            self.assertGreater(hb.elapsed, 0.1)
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)