Add safe file updating

[D-VR]

This supersedes PR #128

Brief overview of the new file updating logic:

• per‑course caches are stored as .syncmymoodle_cache inside each course directory. Each cache contains the full tree of the last full sync for the course (could be split into just a course subtree in the future)
• During downloads, we use the course cache to compare Moodle's timemodified with the previous value. If it hasn't changed, we skip re‑downloading the file
• When Moodle's timemodified changed, we use the cached ETag to detect local modifications: if the local file's content hash no longer matches the old ETag, we treat it as a local change and apply a configured conflict strategy
• We Support configurable conflict handling via update_files_conflict (rename, keep, overwrite), with rename moving modified files to .syncconflict..
• If there is no cache (yet), we compare the current Moodle ETags with the existing local files, and treat matching files as up‑to‑date

closes #77