Added fetch service and a mock Find a Tender server

Fetching:
- Add `fetch service`, which runs a worker per enabled source until
  SIGINT/SIGTERM (internal/fetcher)
- Add util.Worker: runs a task on an interval with a since-checkpoint
  that only advances on success, and a shutdown timeout for in-flight
  work; plus AwaitTask/AwaitTaskE for calls that ignore cancellation
- Add fts_mock and sleeper sources
- Sleeper: sleeps periodically to emulate work being done. Can be
  instructed to block the shutdown signal or handle it cleanly

Mock FTS server (cmd/mock-fts, internal/mock/findATender):
- Serves GET /api/1.0/ocdsReleasePackages with FTS paging, query
  validation, UK local time handling and per-client rate limiting
- Generates fictional releases from an embedded catalog, moving
  procedures through planning, tender and award, and keeps them in a
  JSON Lines store that survives restarts
- Add logging.AccessLog for combined/common access logs

internal/logging/accesslog.go

@@ -0,0 +1,100 @@
+package logging
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// ---------- Access logging ----------
+
+// statusRecorder captures the status code and body size for the access log.
+type statusRecorder struct {
+	http.ResponseWriter
+	status int
+	bytes  int
+}
+
+func (r *statusRecorder) WriteHeader(code int) {
+	if r.status == 0 {
+		r.status = code
+	}
+	r.ResponseWriter.WriteHeader(code)
+}
+
+func (r *statusRecorder) Write(b []byte) (int, error) {
+	if r.status == 0 {
+		r.status = http.StatusOK
+	}
+	n, err := r.ResponseWriter.Write(b)
+	r.bytes += n
+	return n, err
+}
+
+// logField escapes a value for a quoted log field the way nginx does
+// (quotes, backslashes and control characters become \xHH) and uses "-"
+// for empty values.
+func logField(s string) string {
+	if s == "" {
+		return "-"
+	}
+	var b strings.Builder
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c == '"' || c == '\\' || c < 0x20 || c == 0x7f {
+			fmt.Fprintf(&b, "\\x%02X", c)
+		} else {
+			b.WriteByte(c)
+		}
+	}
+	return b.String()
+}
+
+// clientKey attempts to produce a client host name as a key identifier to split
+// requests in the log between sources. Could be extended to also check
+// Forwarded-For or other proxy headers
+func clientKey(req *http.Request) string {
+	host, _, err := net.SplitHostPort(req.RemoteAddr)
+	if err != nil {
+		return req.RemoteAddr
+	}
+	return host
+}
+
+func AccessLog(logger *log.Logger, combined bool, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		start := time.Now()
+		rec := &statusRecorder{ResponseWriter: w}
+		next.ServeHTTP(rec, req)
+		if rec.status == 0 {
+			rec.status = http.StatusOK
+		}
+
+		user := "-"
+		if u, _, ok := req.BasicAuth(); ok && u != "" {
+			user = logField(u)
+		}
+		size := "-" // CLF uses "-" for an empty body
+		if rec.bytes > 0 {
+			size = strconv.Itoa(rec.bytes)
+		}
+		line := fmt.Sprintf(`%s - %s [%s] "%s" %d %s`,
+			clientKey(req),
+			user,
+			start.Format("02/Jan/2006:15:04:05 -0700"),
+			logField(req.Method+" "+req.RequestURI+" "+req.Proto),
+			rec.status,
+			size,
+		)
+		if combined {
+			line += fmt.Sprintf(
+				` "%s" "%s"`, logField(req.Referer()), logField(req.UserAgent()),
+			)
+		}
+		logger.Println(line)
+	})
+}