Harden historical sorter header parsing

The prompts used to arrive at this change since the previous commit

User asked to check the audit log in the current directory before reordering the real mailbox.

Any special observations that may be relevant for version management for this version. Be brief.

The inspected log was a partial real backup run, not a hans audit. Header parsing now skips non-bytes FETCH payloads instead of aborting. verplaats_log.json and plan.md were left unstaged.

verplaats_bestaand.py

@@ -372,6 +372,19 @@ def stap2_bronmappen_routing(session: ImapSession, log: dict, dry: bool):
                 mislukt += 1
                 log_failure(log, "stap2", bron, uid, None, "fetch_header", status, data)
                 continue
+            if not isinstance(raw_header, bytes):
+                mislukt += 1
+                log_failure(
+                    log,
+                    "stap2",
+                    bron,
+                    uid,
+                    None,
+                    "fetch_header_type",
+                    status,
+                    f"expected bytes, got {type(raw_header).__name__}",
+                )
+                continue
 
             msg = email.message_from_bytes(raw_header)
             from_raw = decode_hdr(msg.get("From", ""))
@@ -502,6 +515,19 @@ def stap3_facturen(session: ImapSession, log: dict, dry: bool):
                 mislukt += 1
                 log_failure(log, "stap3", bron, uid, None, "fetch_header", status, data)
                 continue
+            if not isinstance(raw_header, bytes):
+                mislukt += 1
+                log_failure(
+                    log,
+                    "stap3",
+                    bron,
+                    uid,
+                    None,
+                    "fetch_header_type",
+                    status,
+                    f"expected bytes, got {type(raw_header).__name__}",
+                )
+                continue
 
             msg      = email.message_from_bytes(raw_header)
             date_str = msg.get("Date", "")