feat: rewrite chat-handler in Go

Replace Python chat handler with Go for smaller container images.
Uses handler-base Go module for NATS, health, telemetry, and service clients.

- RAG pipeline: embed → Milvus → rerank → LLM
- Streaming response chunks
- Optional TTS synthesis
- Custom response_subject support for companions-frontend

Dockerfile

@@ -1,9 +1,23 @@
-# Chat Handler - Using handler-base
-ARG BASE_TAG=latest
-FROM ghcr.io/billy-davies-2/handler-base:${BASE_TAG}
+# Build stage
+FROM golang:1.25-alpine AS builder
 
 WORKDIR /app
 
-COPY chat_handler.py .
+RUN apk add --no-cache ca-certificates
 
-CMD ["python", "chat_handler.py"]
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-w -s" -o /chat-handler .
+
+# Runtime stage
+FROM scratch
+
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=builder /chat-handler /chat-handler
+
+USER 65534:65534
+
+ENTRYPOINT ["/chat-handler"]