Add configurable fallback to apply series cover to books

[mjoetodd]

Summary

• add a new metadata processing config option: fallbackUseSeriesCoverForBook (default false)
• expose the new option through current and deprecated config API DTOs/update mappers for Komga and Kavita
• wire the setting into MetadataUpdater so that when book covers are being updated, missing book covers can fall back to the series cover image (when series cover updates are also enabled)
• document the new setting in README config examples

Behavior

When fallbackUseSeriesCoverForBook is true and both bookCovers and seriesCovers are enabled, Komf will use the resolved series thumbnail as the book thumbnail whenever a specific book thumbnail is not provided by metadata providers.

Validation

• attempted to compile affected modules, but Gradle failed in this environment with * What went wrong: 25.0.1 before compilation could proceed.

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Codex Task