Initial commit

Generic SRV records checker for happyDomain.

For each SRV record attached to an svcs.UnknownSRV service, the checker
resolves every target and probes reachability:

  - DNS resolution (A/AAAA), CNAME detection (RFC 2782 violation),
    null-target detection (RFC 2782 "service explicitly unavailable")
  - TCP connect to target:port for _tcp SRVs
  - UDP probe for _udp SRVs, using ICMP port-unreachable detection

The checker also publishes TLS endpoints (host, port, SNI) for every
SRV target hitting a well-known direct-TLS port (443, 465, 636, 853,
993, 995, 5061, 5223, …) via the EndpointDiscoverer SDK interface, so
a downstream TLS checker can pick them up.

The HTML report groups records as cards and surfaces the most common
failure scenarios (DNS failure, CNAME target, TCP unreachable,
null-target) at the top with remediation guidance.

Dockerfile

@@ -0,0 +1,17 @@
+FROM golang:1.25-alpine AS builder
+
+ARG CHECKER_VERSION=custom-build
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -tags standalone -ldflags "-X main.Version=${CHECKER_VERSION}" -o /checker-srv .
+
+FROM scratch
+COPY --from=builder /checker-srv /checker-srv
+USER 65534:65534
+EXPOSE 8080
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD ["/checker-srv", "-healthcheck"]
+ENTRYPOINT ["/checker-srv"]