feat: bin/brain/serve.go; search backend is Go not Python (#9)

* feat(brain): HTTP serve from bin/brain/serve.go, default Go search binary.

Move the HTTP package to internal/httpapi. Default backend is
var/bin/brain-search, not Python. bin/serve.go stays as a deprecation shim.

* feat(httpapi): default search backend is var/bin/brain-search.

bin/brain/serve.go is the command; bin/serve.go stays as a tagged
deprecation shim. Tests fail if the default path still names Python.

internal/httpapi/server.go

@@ -0,0 +1,152 @@
+// Package server serves the 2dph brain over HTTP.
+//
+// Async by design: every request runs on its own goroutine, and CPU-heavy
+// searches are serialized through a bounded worker pool (a counting
+// semaphore) so N requests can't spawn N search processes at once.
+//
+// Used by bin/brain/serve.go.
+package httpapi
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"log"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type Searcher interface {
+	Search(ctx context.Context, query string, limit int) ([]byte, error)
+}
+
+type Server struct {
+	searcher  Searcher
+	semaphore chan struct{}
+}
+
+const defaultPort = 8630
+
+func NewServer(searcher Searcher, workers int) http.Handler {
+	return &Server{
+		searcher:  searcher,
+		semaphore: make(chan struct{}, workers),
+	}
+}
+
+func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	switch {
+	case r.URL.Path == "/health":
+		writeJSON(w, http.StatusOK, map[string]any{"status": "ok"})
+	case r.URL.Path == "/search":
+		s.handleSearch(w, r)
+	default:
+		writeJSON(w, http.StatusNotFound, map[string]any{"error": "not found"})
+	}
+}
+
+func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
+	q := strings.TrimSpace(r.URL.Query().Get("q"))
+	if q == "" {
+		writeJSON(w, http.StatusBadRequest, map[string]any{"error": "q required"})
+		return
+	}
+	limit := 10
+	if raw := r.URL.Query().Get("n"); raw != "" {
+		n, err := strconv.Atoi(raw)
+		if err != nil || n < 1 || n > 100 {
+			writeJSON(w, http.StatusBadRequest, map[string]any{"error": "n must be int 1..100"})
+			return
+		}
+		limit = n
+	}
+
+	// Worker pool: block until a slot frees, so burst concurrency still
+	// bounds memory (no unbounded python processes).
+	select {
+	case s.semaphore <- struct{}{}:
+		defer func() { <-s.semaphore }()
+	case <-r.Context().Done():
+		return
+	}
+
+	body, err := s.searcher.Search(r.Context(), q, limit)
+	if err != nil {
+		writeJSON(w, http.StatusGatewayTimeout, map[string]any{"error": err.Error()})
+		return
+	}
+	writeRaw(w, http.StatusOK, body)
+}
+
+func writeJSON(w http.ResponseWriter, code int, obj any) {
+	body, _ := json.Marshal(obj)
+	writeRaw(w, code, body)
+}
+
+func writeRaw(w http.ResponseWriter, code int, body []byte) {
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Content-Length", strconv.Itoa(len(body)))
+	w.WriteHeader(code)
+	w.Write(body)
+}
+
+// brainSearcher shells out to the Go brain-search binary (not Python).
+// A single search is bounded and short-lived; the worker pool keeps at most N live.
+type brainSearcher struct {
+	cmdPath string
+	timeout time.Duration
+}
+
+func (b *brainSearcher) Search(ctx context.Context, query string, limit int) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(ctx, b.timeout)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, b.cmdPath, "--json", "-n", strconv.Itoa(limit), query)
+	out, err := cmd.Output()
+	if err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) {
+			return nil, errors.New("search backend failed: " + strings.TrimSpace(string(exitErr.Stderr)))
+		}
+		return nil, err
+	}
+	return out, nil
+}
+
+func defaultSearchCmd(root string) string {
+	if env := os.Getenv("KB_SEARCH_CMD"); env != "" {
+		return env
+	}
+	return filepath.Join(root, "var", "bin", "brain-search")
+}
+
+// Run starts the HTTP server. Reads env: KB_SEARCH_CMD (default
+// $KB_ROOT/var/bin/brain-search), KB_WORKERS (default 4), KB_PORT (default 8630).
+func Run() {
+	root := os.Getenv("KB_ROOT")
+	searchPath := defaultSearchCmd(root)
+	workers := 4
+	if raw := os.Getenv("KB_WORKERS"); raw != "" {
+		if n, err := strconv.Atoi(raw); err == nil && n > 0 {
+			workers = n
+		}
+	}
+	port := defaultPort
+	if raw := os.Getenv("KB_PORT"); raw != "" {
+		if n, err := strconv.Atoi(raw); err == nil && n > 0 {
+			port = n
+		}
+	}
+
+	searcher := &brainSearcher{cmdPath: searchPath, timeout: 60 * time.Second}
+	handler := NewServer(searcher, workers)
+	addr := "127.0.0.1:" + strconv.Itoa(port)
+	log.Printf("serve: %s (workers=%d cmd=%s)", addr, workers, searchPath)
+	if err := http.ListenAndServe(addr, handler); err != nil {
+		log.Fatal(err)
+	}
+}