fix(delete): run delete hooks and queries in one transaction

- resolvespec/restheadspec: single and batch delete use one transaction
- add sqlmock tests for delete transaction behaviour
- testmodels: serial integer ids; update tests accordingly
- add compose testserver, smoke script, podman-first Makefile targets

docker/Dockerfile.testserver

@@ -0,0 +1,13 @@
+FROM golang:1.25-alpine AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -o /out/testserver ./cmd/testserver
+
+FROM alpine:3.20
+RUN apk add --no-cache ca-certificates
+COPY --from=build /out/testserver /usr/local/bin/testserver
+COPY docker/testserver.config.yaml /etc/resolvespec/config.yaml
+EXPOSE 8080
+ENTRYPOINT ["testserver"]