Show a determinate progress bar during export

The export loop can run for a while on large lists. It already knows the
message count up front, so draw a braille spinner + progress bar (frames
and blue borrowed from warez) from inside the Python loop. Written to
stderr so it never pollutes the captured MM3HK| stdout, gated on
isatty() so cron/piped runs stay silent, throttled to ~10 fps, cleared
in finally. Progress bytes never touch the mbox, so its SHA-256 is
unaffected.

mm3-hk-export

@@ -73,7 +73,7 @@ EOF
 }
 
 result=$(mailman-web shell <<EOF
-import sys, os, tempfile, gzip, hashlib
+import sys, os, tempfile, gzip, hashlib, time
 from hyperkitty.models import MailingList
 addr = "$addr"
 GZIP = $gzip_py
@@ -96,14 +96,35 @@ raw = os.fdopen(fd, "w+b")
 # mtime=0 keeps the gzip header deterministic (no timestamp), matching the
 # web export, which uses a bare zlib gzip stream.
 fh = gzip.GzipFile(fileobj=raw, mode="wb", mtime=0) if GZIP else raw
+# Live progress bar on stderr: stdout is captured by the shell (the MM3HK|
+# line), so a shell-side spinner can't see per-message progress, but this loop
+# knows n up front. TTY-only, so cron/piped runs stay silent. Braille frames
+# and blue borrowed from warez's spinner.
+tty = sys.stderr.isatty()
+SPIN = "⠋⠙⠹⠸⠼⠴⠦⠧⠇⠏"
+def draw(done, spin):
+    w = 24
+    fill = done * w // n
+    sys.stderr.write("\r\033[0;34m%s\033[0m [%s%s] %d/%d (%d%%)" % (
+        SPIN[spin % len(SPIN)], "█" * fill, "░" * (w - fill),
+        done, n, done * 100 // n))
+    sys.stderr.flush()
+spin = 0
+last = 0.0
 try:
-    for e in ml.emails.order_by("archived_date").iterator():
+    for i, e in enumerate(ml.emails.order_by("archived_date").iterator(), 1):
         fh.write(e.as_bytes())
         fh.write(b"\n\n")
+        if tty:
+            now = time.monotonic()
+            if now - last >= 0.1 or i == n:  # ~10 fps, plus a final 100% frame
+                draw(i, spin); spin += 1; last = now
 finally:
     if GZIP:
         fh.close()  # flush the gzip trailer into raw (does not close raw)
     raw.flush()
+    if tty:
+        sys.stderr.write("\r\033[K"); sys.stderr.flush()  # clear the bar
 # Hash the on-disk bytes via the held fd, then hand root the digest so it can
 # verify the file wasn't swapped/truncated by another same-uid process before
 # it copies it. (A fully-compromised exporter is out of scope, since it controls