Add Dockerfile and .dockerignore

Multi-stage build: golang:1.24-alpine builder, alpine:3.21 runtime.
Verified locally: all fx modules instantiate, migrations run,
healthcheck responds at /.well-known/healthcheck.json.

Dockerfile

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+FROM golang:1.24-alpine AS builder
+
+RUN apk add --no-cache git
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+ARG VERSION=dev
+RUN go build -ldflags "-X main.Version=${VERSION}" -o /chatd ./cmd/chatd
+
+FROM alpine:3.21
+
+RUN apk add --no-cache ca-certificates
+COPY --from=builder /chatd /usr/local/bin/chatd
+
+WORKDIR /data
+EXPOSE 8080
+
+ENTRYPOINT ["chatd"]