fix: bound media search and backfill requests

[marinofaggiana]

Summary

Fixes #4255 by replacing effectively unbounded media WebDAV searches with bounded requests.

This PR introduces two changes:

• Media verification now uses a fixed search limit of 1,000 results.
• Media metadata backfill now processes bounded batches of up to 1,000 results, split into four pages of 250.

Backfill behavior

• Persists the oldest successfully processed date as a Realm cursor.
• Checkpoints progress after every completed page.
• Resumes from the saved date during the next background task.
• Restarts safely from the beginning when migrating an incomplete legacy offset.
• Cancels the underlying HTTP request when the background task expires.
• Preserves correct pagination when hidden files are present.
• Detects a non-advancing date cursor and stops instead of looping indefinitely.
• Bumps the Realm schema version to 414.

Known limitation

Nextcloud WebDAV SEARCH supports sorting by displayname, but does not support range comparisons such as lte for that field. Pagination therefore cannot use a composite date-and-filename cursor.

If at least 1,000 media items share the exact same cursor date, the backfill stops safely instead of entering an infinite loop. A future background task may encounter the same boundary again, but existing items remain protected by ocId deduplication.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI