Handle SQLite cache errors gracefully instead of aborting upload

[caglarpir]

Summary:

• Fix sqlite3.OperationalError: disk I/O error in the file handle cache from crashing the entire upload process

  • PersistentCache.get() now catches sqlite3.DatabaseError and returns None (cache miss) instead of propagating the exception
  • Cache failures are logged as warnings so they remain visible for debugging

  Context

  The upload file handle cache is a SQLite database that stores server-returned handles for already-uploaded images. It is purely an optimization to skip re-uploading — if the cache is
  unavailable, the server's fetch_offset endpoint confirms prior uploads at the cost of one extra HTTP round-trip per image.

  Previously, any SQLite error during cache lookup (disk I/O error, corrupt database, etc.) propagated up through _continue_or_fail which treated it as a fatal error, aborting the upload of all
  remaining images. This was observed on Windows with 4 concurrent upload workers rapidly opening/closing SQLite connections to the same cache file.