BUG: O(n²) bubble sort in fetcher batch ordering

[andrinoff]

Describe the bug

fetcher/fetcher.go:505-511 sorts each batch by UID using nested loops:

for i := 0; i < len(batchEmails); i++ {
    for j := i + 1; j < len(batchEmails); j++ {
        if batchEmails[j].UID > batchEmails[i].UID {
            batchEmails[i], batchEmails[j] = batchEmails[j], batchEmails[i]
        }
    }
}

Quadratic on every batch. The default chunk size is 50 so it's not catastrophic, but with a larger limit it becomes the dominant cost of the fetch.

Expected behavior

Replace with sort.Slice(batchEmails, func(i, j int) bool { return batchEmails[i].UID > batchEmails[j].UID }).

[Haroka-74]

/assign