email: fix backfill crash on emails with no Reply-To; Sync now retries errored mailboxes (v0.1.0:62)

insert_email's recipients loop did `for a in parsed.get(kind, [])`, but the parser sets
reply_to=None when there is no Reply-To header, so .get returns None (key present) and the
loop raised 'NoneType' object is not iterable — aborting the entire Gmail backfill on the
first such email (i.e. almost immediately). Fixed with `or []`. Regression test
test_insert_email.py (reply_to=None, all-None recipients, happy path).

Because the scheduler intentionally skips error-status accounts (no retry storms), an
errored mailbox would never resume on its own. "Sync now" now clears error status first,
so it is an explicit retry; backfill resumes from its saved cursor and dedups by
Message-ID, so nothing is re-captured.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/email_integration/routes.py

@@ -377,6 +377,17 @@ def _h_run_now(handler):
     # Reuse existing rate limit so admins can't hammer this.
     if handler.rate_limited("email-sync-now", 6):
         return handler.send_error_json("Too many requests", 429)
+    # A manual sync is an explicit retry. The scheduler intentionally skips
+    # error-status accounts (no retry storms), so clear that status here so a
+    # mailbox that previously errored is re-attempted. Backfill resumes from its
+    # saved cursor and dedups by Message-ID, so nothing is re-captured twice.
+    conn = _conn()
+    try:
+        conn.execute("UPDATE email_accounts SET sync_status = 'pending', sync_error = NULL "
+                     "WHERE sync_enabled = 1 AND sync_status = 'error'")
+        conn.commit()
+    finally:
+        conn.close()
     result = _sched.trigger_run_now()
     handler.send_json(result)