import: keep what an interrupted IMAP or Maildir import wrote

IMAP import wrote a whole folder in one transaction and stopped the folder
at the first message it could not import. A crash near the end of a large
INBOX kept nothing, and one bad INTERNALDATE lost the rest of the folder.
Worse, when a folder stopped early, fetches still in flight for it could be
filed into the next folder's mailbox.

- The transaction is committed after every fetch chunk. Each message is
  written in its own savepoint, so what is committed is always whole, and a
  rerun fetches only the UIDs still missing.
- A message that cannot be imported is rolled back on its own, logged with
  its folder and UID, and counted as failed; the folder carries on, and the
  message stays out of the UID map so the next run tries it again. Archive
  and I/O errors still stop the run.
- Fetch jobs and events carry a folder generation. Moving to a new folder
  cancels queued work for older ones, and any event from an older
  generation is dropped, never filed. Shutdown drains in-flight events
  before joining the workers, so it cannot hang on a blocked worker.
- INTERNALDATE month names are matched in any case.
- Maildir import gets the same per-message savepoint, and commits every 500
  new messages instead of once per folder.

src/sync/import_imap/internaldate.rs

@@ -1,5 +1,6 @@
 /*
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <[email protected]>
+ * SPDX-FileCopyrightText: 2026 John Coffey <[email protected]>
  *
  * SPDX-License-Identifier: Apache-2.0 OR MIT
  */
@@ -58,20 +59,22 @@ pub fn imap_internaldate_to_rfc3339(s: &str) -> Result<String, Error> {
     Ok(out)
 }
 
+// RFC 3501 spells the month "Jan", but servers are not all that careful,
+// and a date is not worth losing a message over: match any case.
 fn month_to_num(s: &str) -> Result<u32, Error> {
-    let m = match s {
-        "Jan" => 1,
-        "Feb" => 2,
-        "Mar" => 3,
-        "Apr" => 4,
-        "May" => 5,
-        "Jun" => 6,
-        "Jul" => 7,
-        "Aug" => 8,
-        "Sep" => 9,
-        "Oct" => 10,
-        "Nov" => 11,
-        "Dec" => 12,
+    let m = match s.to_ascii_lowercase().as_str() {
+        "jan" => 1,
+        "feb" => 2,
+        "mar" => 3,
+        "apr" => 4,
+        "may" => 5,
+        "jun" => 6,
+        "jul" => 7,
+        "aug" => 8,
+        "sep" => 9,
+        "oct" => 10,
+        "nov" => 11,
+        "dec" => 12,
         other => return Err(Error::Partial(format!("INTERNALDATE month {other:?}"))),
     };
     Ok(m)
@@ -99,6 +102,22 @@ fn parse_zone(s: &str) -> Result<(char, u32, u32), Error> {
 mod tests {
     use super::*;
 
+    #[test]
+    fn month_matches_any_case() {
+        for d in [
+            "12-May-2025 10:00:00 +0000",
+            "12-may-2025 10:00:00 +0000",
+            "12-MAY-2025 10:00:00 +0000",
+        ] {
+            assert_eq!(
+                imap_internaldate_to_rfc3339(d).unwrap(),
+                "2025-05-12T10:00:00Z",
+                "{d}"
+            );
+        }
+        assert!(imap_internaldate_to_rfc3339("12-Mai-2025 10:00:00 +0000").is_err());
+    }
+
     #[test]
     fn utc_zone_becomes_z() {
         assert_eq!(