fix: prevent completed paths exceeding filesystem limits

[calmcacil]

Problem

Long download display names can produce completed directory names longer than
the filesystem's per-component limit. On Linux filesystems such as ext4, this
can cause finalization to fail when TorBoxarr creates or renames the completed
directory.

Cause

CompletedPathForJob used the sanitized display name and job ID directly as a
single path component without applying a length limit.

Fix

• Limit the completed directory component to 255 bytes.
• Preserve the job ID suffix.
• Avoid cutting a UTF-8 character in half.
• Add regression coverage for a long real-world display name.

Validation

• go test -count=1 ./...
• go vet ./...
• git diff --check

[calmcacil]

For context, this was triggered by an anime release with the title english (japanese) - etc — an absolute menace of a release name. Combined with TorBoxarr’s job ID suffix, the resulting completed directory exceeded the filesystem’s per-component limit during finalization. The regression test is based on a similarly long real-world title.

[MrJoiny]

Thank you, great job.