feat: HTTPException for non-500 JSON responses (#48)

[ZhuchkaTriplesix]

• Add oxyroute.exceptions.HTTPException and Rust dispatch path via _http_exception_payload + send_python_error before generic 500
• Tests: httpx ASGI 404/422/headers; ValueError still 500
• docs/handlers.md and feature.md; PRIORITIES: feat: HTTP 500 JSON for handler/dependency errors (Closes #3) #23/feat: global HTTPException and exception handlers #48 done

Closes #48

Summary

• What this PR does (1–3 sentences).
• If it closes a ticket: Closes #N (replace N).

Base branch

• This PR targets dev, not main (unless maintainers asked for a hotfix).

Checklist

• cargo build (and cargo clippy if you touched Rust)
• maturin develop + tests as in docs/development.md (e.g. pytest from a clean cwd / installed wheel)
• No unrelated drive-by refactors; commits are atomic and scoped

Note on .github/ISSUE_BACKLOG/

If you only touch issue template files under .github/ISSUE_BACKLOG/ (not application code), say so in the summary. Prefer a separate PR for backlog-only edits when possible.

[ZhuchkaTriplesix]

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