feat: implement T22, T18, T23 — dev stack, profile tests, server binary

- T22: docker-compose.dev.yml dev stack, Dockerfile, root Makefile
- T18: Profile test suite (Scenario A) — 8 integration tests with real handlers
- T23: Server binary wiring all components, config validation, /healthz
- Config: ValidateConfig with startup validation

14 test packages pass.

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

Dockerfile

@@ -0,0 +1,11 @@
+FROM golang:1.23-alpine AS builder
+WORKDIR /app
+COPY src/go.mod src/go.sum ./
+RUN go mod download
+COPY src/ .
+RUN CGO_ENABLED=0 go build -o keycape ./cmd/keycape
+
+FROM gcr.io/distroless/static-debian12
+COPY --from=builder /app/keycape /keycape
+EXPOSE 8080
+ENTRYPOINT ["/keycape"]