Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/logging/accesslog.go

@@ -19,19 +19,19 @@ type statusRecorder struct {
 	bytes  int
 }
 
-func (r *statusRecorder) WriteHeader(code int) {
-	if r.status == 0 {
-		r.status = code
+func (s *statusRecorder) WriteHeader(code int) {
+	if s.status == 0 {
+		s.status = code
 	}
-	r.ResponseWriter.WriteHeader(code)
+	s.ResponseWriter.WriteHeader(code)
 }
 
-func (r *statusRecorder) Write(data []byte) (int, error) {
-	if r.status == 0 {
-		r.status = http.StatusOK
+func (s *statusRecorder) Write(data []byte) (int, error) {
+	if s.status == 0 {
+		s.status = http.StatusOK
 	}
-	written, err := r.ResponseWriter.Write(data)
-	r.bytes += written
+	written, err := s.ResponseWriter.Write(data)
+	s.bytes += written
 	return written, err
 }
 
@@ -42,6 +42,7 @@ func logField(value string) string {
 	if value == "" {
 		return "-"
 	}
+
 	var builder strings.Builder
 	for i := 0; i < len(value); i++ {
 		char := value[i]
@@ -51,16 +52,17 @@ func logField(value string) string {
 			builder.WriteByte(char)
 		}
 	}
+
 	return builder.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)
+func clientKey(r *http.Request) string {
+	host, _, err := net.SplitHostPort(r.RemoteAddr)
 	if err != nil {
-		return req.RemoteAddr
+		return r.RemoteAddr
 	}
 	return host
 }
@@ -68,19 +70,21 @@ func clientKey(req *http.Request) string {
 // AccessLog wraps next, writing a line to logger for each request in the
 // Apache/nginx Common Log Format, or the Combined Log Format (which adds the
 // referer and user agent) if combined is true.
-func AccessLog(logger *log.Logger, combined bool, next http.Handler) http.Handler {
-	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+func AccessLog(
+	logger *log.Logger, combined bool, next http.Handler,
+) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		start := time.Now()
 		recorder := &statusRecorder{
 			ResponseWriter: w,
 		}
-		next.ServeHTTP(recorder, req)
+		next.ServeHTTP(recorder, r)
 		if recorder.status == 0 {
 			recorder.status = http.StatusOK
 		}
 
 		user := "-"
-		if username, _, ok := req.BasicAuth(); ok && username != "" {
+		if username, _, ok := r.BasicAuth(); ok && username != "" {
 			user = logField(username)
 		}
 		size := "-" // CLF uses "-" for an empty body
@@ -89,16 +93,16 @@ func AccessLog(logger *log.Logger, combined bool, next http.Handler) http.Handle
 		}
 		line := fmt.Sprintf(
 			`%s - %s [%s] "%s" %d %s`,
-			clientKey(req),
+			clientKey(r),
 			user,
 			start.Format("02/Jan/2006:15:04:05 -0700"),
-			logField(req.Method+" "+req.RequestURI+" "+req.Proto),
+			logField(r.Method+" "+r.RequestURI+" "+r.Proto),
 			recorder.status,
 			size,
 		)
 		if combined {
 			line += fmt.Sprintf(
-				` "%s" "%s"`, logField(req.Referer()), logField(req.UserAgent()),
+				` "%s" "%s"`, logField(r.Referer()), logField(r.UserAgent()),
 			)
 		}
 		logger.Println(line)