Fix backend-core TARGETARCH: auto-detect instead of hardcoded arm64

The Dockerfile hardcoded TARGETARCH=arm64 for Mac Mini. Coolify server
is x86_64, causing exit code 126 (wrong binary arch). Now uses Docker
BuildKit's auto-detected TARGETARCH with dpkg fallback.

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

backend-core/Dockerfile

@@ -43,11 +43,12 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 # Install DevSecOps tools (gitleaks, trivy, grype, syft)
-ARG TARGETARCH=arm64
+ARG TARGETARCH
 RUN set -eux; \
+    ARCH="${TARGETARCH:-$(dpkg --print-architecture)}"; \
     # Gitleaks
     GITLEAKS_VERSION=8.21.2; \
-    if [ "$TARGETARCH" = "arm64" ]; then GITLEAKS_ARCH=arm64; else GITLEAKS_ARCH=x64; fi; \
+    if [ "$ARCH" = "arm64" ]; then GITLEAKS_ARCH=arm64; else GITLEAKS_ARCH=x64; fi; \
     curl -sSfL "https://github.com/gitleaks/gitleaks/releases/download/v${GITLEAKS_VERSION}/gitleaks_${GITLEAKS_VERSION}_linux_${GITLEAKS_ARCH}.tar.gz" \
       | tar xz -C /usr/local/bin gitleaks; \
     # Trivy