chore(deps): update rust docker tag to v1.91 (#310)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Dockerfile

@@ -8,7 +8,7 @@ COPY frontend ./
 RUN npm run build
 
 # --- Backend build stage ---
-FROM rust:1.90-bookworm as backend-builder
+FROM rust:1.91-bookworm as backend-builder
 
 # Install system dependencies for OCR and PDF processing
 RUN apt-get update && apt-get install -y \

Dockerfile.dev

@@ -1,5 +1,5 @@
 # Development Dockerfile with hot-reloading support for Rust backend
-FROM rust:1.90-bookworm
+FROM rust:1.91-bookworm
 
 # Install system dependencies for OCR and PDF processing
 RUN apt-get update && apt-get install -y \

Dockerfile.stress

@@ -1,5 +1,5 @@
 # Multi-stage Dockerfile for WebDAV Stress Testing
-FROM rust:1.90-alpine AS builder
+FROM rust:1.91-alpine AS builder
 
 # Install build dependencies
 RUN apk add --no-cache \