Preserve worker error backtraces

[reneklacan]

Summary

• store worker failures with Debug formatting by default so captured backtraces reach retry and dead-job metadata
• add a runtime error formatter override for applications that need a custom representation
• cover default and custom formatting in integration tests and document backtrace requirements

Verification

• cargo fmt --all -- --check
• cargo clippy --all-features --workspace --all-targets -- -D warnings
• cargo test --workspace --all-features
• cargo check --all-features --workspace

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Note

Low Risk
Stored error text changes from Display to Debug by default, which may alter dashboard copy but does not touch auth or persistence semantics; optional backtraces can expose sensitive stack data if enabled.

Overview
Worker failures persisted on retry and dead jobs now use each error’s Debug string instead of Display, so types that embed backtraces (when enabled) surface that detail in stored metadata and the dashboard.

Adds RuntimeBuilder::error_formatter and RuntimeSettings::format_error so apps can override the stored text (e.g. back to error.to_string()). The executor and queue drain path both route failures through this helper; drain now takes runtime settings for formatting.

Changelog, README (backtrace env notes and sensitivity), and integration tests cover default Debug output and custom formatters on dead, retry, and drain flows.

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