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/db.py

@@ -188,7 +188,10 @@ def insert_email(conn: sqlite3.Connection, *, parsed: dict, match_status: str) -
         elif kind == "reply_to" and parsed.get("reply_to"):
             addrs = [(parsed["reply_to"], None)]
         else:
-            for a in parsed.get(kind, []):
+            # `or []` (not get(kind, [])): the key is often present with value None
+            # (e.g. reply_to when there is no Reply-To header), and `for a in None`
+            # would raise TypeError and abort the whole backfill.
+            for a in (parsed.get(kind) or []):
                 if isinstance(a, dict):
                     addrs.append((a.get("email"), a.get("name")))
                 else: