Fix: Use Rust latest (1.85+) for edition2024 support

Our dependencies (time-core, getrandom, uuid, etc.) now require
Rust 1.85+ for edition2024 support.

Changes:
- Dockerfile.dev: Updated to rust:latest
- Dockerfile.prod: Updated to rust:latest for builder stage

This resolves the edition2024 compilation errors.

backend/docker/Dockerfile.dev

@@ -1,4 +1,4 @@
-FROM rust:1.83-alpine
+FROM rust:latest
 WORKDIR /app
 RUN apk add --no-cache musl-dev pkgconf openssl-dev curl wget git pkgconfig
 COPY Cargo.toml ./