Update rust Docker tag to v1.97

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
rust (source)	stage	minor	1.91 → 1.97

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[doubleword-code]

Summary

This PR updates the Rust Docker builder image from rust:1.91 to rust:1.96. This is a straightforward dependency bump that brings the build environment up to the latest stable Rust version (1.96.0 was released May 28, 2026). The change is minimal, low-risk, and includes security fixes from Cargo (CVE-2026-5222, CVE-2026-5223).

Verdict: Ready to approve — no issues identified.

Research notes

• Docker Hub: Confirmed rust:1.96 and rust:1.96.0 tags exist and were pushed recently.
• Rust 1.96.0 changelog: Fetched from releases.rs — no breaking changes affecting this project's dependencies. Notable inclusions: stabilized assert_matches! macros and Cargo security patches.
• Project dependencies: All crates (tokio 1.42, axum 0.7, reqwest 0.12, serde 1.0, etc.) are compatible with Rust 1.96 on edition 2021.
• Codebase scan: No other references to Rust version found (no rust-toolchain.toml, no CI version matrix).

Suggested next steps

1. Merge this PR — it's a safe, beneficial update.
2. Consider verifying the built container image runs successfully after merge (standard smoke test).

General findings

None — the change is exactly what it claims to be and introduces no regressions.