fix: bump Rust image to 1.85 for edition2024 support

The time crate v0.3.47 requires Rust edition 2024 which needs
Cargo 1.85+.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: EstrellaXD

Dockerfile

@@ -6,7 +6,7 @@ RUN npm ci
 COPY frontend/ .
 RUN npm run build
 
-FROM rust:1.83-slim AS backend
+FROM rust:1.85-slim AS backend
 
 WORKDIR /build
 

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM rust:1.83-slim AS builder
+FROM rust:1.85-slim AS builder
 
 WORKDIR /build