Bump Rust image to 1.86 for edition2024 support

getrandom 0.4.1 requires edition2024 which needs Cargo >= 1.85.

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

Dockerfile

@@ -7,7 +7,7 @@ COPY web/ ./
 RUN npm run build
 
 # Stage 2: Build backend
-FROM rust:1.84-alpine AS backend
+FROM rust:1.86-alpine AS backend
 RUN apk add --no-cache musl-dev
 WORKDIR /app
 COPY Cargo.toml Cargo.lock ./