feature: multi-part m4b merging

[benjitobz]

Description

Chaptarr already merges multi-part audiobook downloads into a single M4B when conversion is planned, but releases that arrive as multiple M4B parts are skipped entirely — since no format conversion was needed. This adds an opt-in "Create Single File" setting on quality profiles: when enabled, multi-part downloads whose parts are already M4B are merged into one M4B file on import. Embedded chapters from each part are preserved and re-offset in the merged file, so chapter navigation survives the concatenation instead of being lost. With the setting off (the default), behavior is unchanged. Reason for adding this is it's nice to have the books in one file, especially while using ABS in a vehicle.

Fixes # — n/a (feature)

Database Migration

YES - Migration 103_add_quality_profile_merge_multi_part_files:

• Adds a MergeMultiPartFiles boolean column (not-null, default false) to the QualityProfiles table, guarded by an existence check.
• The fresh-install baseline schema (001_chaptarr_complete_schema) is updated to include the same column.

How was this tested?

Docker (linux/amd64) on an Ubuntu server host, image built with Dockerfile.build. Exercised with real multi-part audiobook releases:

• With "Create Single File" enabled on the profile: multi-part M4B releases are merged into a single M4B on import, and the merged file's chapters were verified present and correctly offset across part boundaries.
• With the setting disabled: multi-part M4B releases import unchanged (parts kept), confirming the gate defaults to existing behavior.
• Existing multi-part MP3 → M4B conversion paths re-tested to confirm they're unaffected.
• Migration verified on an existing database (column added, profiles default to off) and on a fresh install via the baseline schema.

Screenshots (UI changes only) - (red box is just show the change not actually present in the UI)

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