Merge multi-part M4B downloads into a single file

[benjitobz]

Description

Multi-part downloads that already consist of M4B files were the one case the M4B conversion pipeline did not create 1 m4b file. There was a guard in ImportApprovedBooks which skipped conversion entirely, so a 10-part M4B audiobook imported as 10 separate files while multi-part MP3/M4A/FLAC merged into one. This was obviously intentional but for some setups a single m4b file works better.

This adds an opt-in per-quality-profile setting, MergeMultiPartFiles, exposed as a "Create Single File" checkbox below Convert To in the quality profile editor. When enabled, multi-part all-M4B downloads are routed through the existing m4b-tool merge pipeline like any other format.

• Single-file M4B downloads are still imported untouched to avoid a pointless re-encode.
• GetPlannedConversionTarget is unchanged, so queue display and decision-engine effective-quality comparisons behave exactly as before; the new ShouldMergeMultiPartM4b helper gates the import path only.
• The flag round-trips through the API resource, openapi.json, and settings backups.

Database Migration

YES.
Migration 103 adds a MergeMultiPartFiles boolean column (NOT NULL, default false) to QualityProfiles, with an existence check Migration 001 complete-schema migration gains the same column for fresh installs.

How was this tested?

Built with docker build -f Dockerfile.build on Ubuntu and deployed over a live SQLite install (v0.9.936.0) that previously ran the base branch. Verified on startup:

• Migration 103 applied cleanly to the existing database and the app started without errors.
• GET /api/v1/qualityprofile returns mergeMultiPartFiles on existing profiles (defaulting to false).
• The checkbox renders and persists in the profile editor.

Screenshots (UI changes only)