Phase 2a: kamado-api Go middleware (core MVP)

Go 1.22 module that polls CKPool's Unix socket control API, queries
Bitcoin Core over JSON-RPC, merges both into a thread-safe snapshot, and
serves it over REST. Layout:

  api/
  ├── cmd/kamado-api/main.go              signal-aware entrypoint
  └── internal/
      ├── config/       env var loader with validation
      ├── ckpool/       socket client (4-byte LE length-prefixed wire
      │                 protocol verified against libckpool.c), typed
      │                 response models for poolstats/users/workers/
      │                 clients/uptime, + unit tests using a fake
      │                 unix socket server
      ├── bitcoind/     minimal JSON-RPC client, getblockchaininfo
      │                 and getnetworkhashps
      ├── state/        Aggregator that refreshes a merged Snapshot
      │                 on a ticker; readers get a copy under RWMutex
      └── httpapi/      REST handlers on Go 1.22 ServeMux:
                        /api/health /api/pool /api/users
                        /api/workers /api/clients /api/snapshot

CKPool stores hashrate as "dsps" (diff shares per second); we convert
to H/s via the 2^32 constant used by GoBrrr-Pool and other clients.
Every stat CKPool exposes to its socket API is surfaced — useragent,
IP, per-client diff, per-worker best diff — closing the gap against
Bassin which only reads the 60-second stats files.

Dockerfile does a CGO_ENABLED=0 static build on golang:1.22-bookworm
with -trimpath -ldflags=-s -w. docker-compose now runs both ckpool and
kamado-api, sharing a named volume for /run/ckpool so the API can
dial the stratifier socket directly.

Deferred to Phase 2b (called out in README):
  - Bitcoin Core ZMQ hashblock subscriber
  - WebSocket push for real-time UI updates
  - SQLite persistence (block history, best-share history)
  - CKPool log tailer for "Solved and confirmed block" detection

Tests and build NOT run in this commit — Go isn't installed in the
dev environment. Run `make api-test` or `make api` to verify.

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

api/Dockerfile

@@ -0,0 +1,34 @@
+# ============================================================================
+# kamado-api build stage
+# ============================================================================
+FROM golang:1.22-bookworm AS build
+
+WORKDIR /src
+# Cache deps first
+COPY go.mod ./
+RUN go mod download 2>/dev/null || true
+COPY . .
+
+# Static build — CGO off, stripped, reproducible-ish
+RUN CGO_ENABLED=0 GOOS=linux go build \
+        -trimpath \
+        -ldflags="-s -w" \
+        -o /out/kamado-api \
+        ./cmd/kamado-api
+
+# ============================================================================
+# Runtime: distroless-ish minimal
+# ============================================================================
+FROM debian:bookworm-slim AS runtime
+
+RUN apt-get update && apt-get install --no-install-recommends -y \
+        ca-certificates \
+        tini \
+    && rm -rf /var/lib/apt/lists/* \
+    && mkdir -p /var/lib/kamado /var/log/ckpool /run/ckpool
+
+COPY --from=build /out/kamado-api /usr/local/bin/kamado-api
+
+EXPOSE 8080
+
+ENTRYPOINT ["/usr/bin/tini", "--", "/usr/local/bin/kamado-api"]