feat: add multi-architecture Docker build setup

- Add Docker BuildKit builder setup for AMD64 and ARM64 platforms
- Update backend and frontend Dockerfiles with platform flags
- Create comprehensive build scripts for multi-arch workflows
- Add verification script to test multi-architecture images
- Update Makefile with multi-arch Docker targets
- Add detailed documentation for multi-architecture setup

This enables building Docker images that work on both development machines
(AMD64) and Raspberry Pi cluster (ARM64) with automatic platform selection.

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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/Dockerfile

@@ -1,5 +1,5 @@
-# Multi-stage build for ARM compatibility (Raspberry Pi)
-FROM oven/bun:1-alpine AS builder
+# Multi-stage build for multi-architecture support (AMD64, ARM64)
+FROM --platform=$BUILDPLATFORM oven/bun:1-alpine AS builder
 
 WORKDIR /app
 
@@ -13,7 +13,7 @@ RUN bun install --frozen-lockfile --production
 COPY . .
 
 # --- Production stage ---
-FROM oven/bun:1-alpine
+FROM --platform=$TARGETPLATFORM oven/bun:1-alpine
 
 # Install curl for health checks and other utilities
 RUN apk add --no-cache curl wget