Fix clippy regressions and add AppCode parser

[RAprogramm]

Summary

• implement ParseAppCodeError and FromStr for AppCode, re-export the parser, and cover it with unit tests
• fix clippy warnings across axum/sqlx/tonic adapters and document the intentional AppError return size in ErrorResponse::with_details
• bump the crate to v0.20.4, refresh README examples, and record the changes in the changelog

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d377d4a088832bb1ffdec9a3331dd6