Automatically remove server cache for non-existing loom-files

[JobLeonard]

Split from #156. To quote the reasoning there:

When iterating over a project folder, it might be prudent to delete any extracted metadata without a matching loom file.

• ✔️ pro: automatically cleans up stale cache
• ✔️ pro: avoids issue of renaming a loom file, later replacing it with another loom file, and the new loom file being stuck with old stale cache
• ❌ con: does not prevent previous when immediately replacing a loom file.
  • ✔️ Counterargument: this is the kind of "expert mode" renaming where one should realise the cache needs to be updated.
    • Add to documentation?
• ❌ con: when debugging, I sometimes rename metadata to a backup, then change code, then refresh to see if changes in code reflect changes to cache.
  • ✔️ Counterargument: specific niche debugging scenario,
  • ✔️ Counterargument: different renaming scheme (FILENAME.loom.old_file_md_1.json.gzip instead of FILENAME_old1.loom.file_md.json.gzip) fixes this.
  • ✔️ Counterargument: as an alternate workaround, not auto-cleaning in debug mode will avoid this issue (and is likely useful for debugging anyway)

Verdict: should be implemented