Fix Prisma OpenSSL error on Alpine Linux

- Add binaryTargets = ["native", "linux-musl-openssl-3.0.x"] to the
  Prisma generator so the correct engine binary is bundled for Alpine
- Install openssl in the runtime Docker stage via apk

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

docker/backend.Dockerfile

@@ -21,6 +21,9 @@ RUN npx tsc
 # ---- Runtime stage ----
 FROM node:20-alpine AS runner
 
+# Prisma needs libssl on Alpine (musl)
+RUN apk add --no-cache openssl
+
 WORKDIR /app
 
 # Copy only production runtime artifacts