M4B audiobook parsing improvements

[chocolatkey]

This is a follow-up to the audiobook parsing that was implemented. Some weak points were identified after testing it, in particular with large .m4b files. More concurrency is now used to load the TOC of such files more quickly.
A bug was also found in the file fetcher
(Most changes by Claude Fable)

[Copilot]

Pull request overview

This PR improves M4B/MP4 audiobook probing performance for large files by adding bounded intra-file concurrency when fetching scattered chapter-title samples, and fixes an S3 range-read bug in the fetcher layer.

Changes:

• Thread a concurrency setting from AudioParser through probing to MP4 chapter extraction to parallelize scattered chapter-title reads.
• Implement concurrent, coalesced MP4 chapter-title sample fetching using errgroup with a configurable limit.
• Fix s3Resource.Read to actually send the constructed GetObjectInput (including the Range header), and add tests to verify ranged vs whole-object reads.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/parser/audio/rich.go	Passes parser concurrency into per-item probing.
pkg/parser/audio/probe.go	Threads concurrency into MP4 probing path.
pkg/parser/audio/parser.go	Clarifies concurrency semantics (file-level and intra-file).
pkg/parser/audio/mp4.go	Adds concurrent chapter-title sample fetching with bounded parallelism.
pkg/fetcher/fetcher_s3.go	Fixes S3 ranged reads by using the request object with Range applied.
pkg/fetcher/fetcher_s3_test.go	Adds coverage to assert correct S3 range-header behavior and response slicing.
pkg/fetcher/fetcher_file.go	Makes file-backed resources safer for concurrent ranged reads (ReadAt/SectionReader) and serializes offset-based reads.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.