refactor: modularize codebase — deduplicate, extract, clean up

- Unify duplicate uTLS transports into shared internal/transport package
- Extract shared version constant into internal/version
- Move LoadDefaultCredentials from config to auth (remove config→auth import)
- Deduplicate handler.go: extract telemetry/error helpers (324→268 lines)
- Break up main.go::run() into initCredential/initEmbedded
- Eliminate logging.Config duplication (use config.LoggingConfig directly)
- Extract logWriter to embedded/log.go, SSE fixtures to consts in sniff.go
- Use uTLS client for usage polling (consistent TLS fingerprint)
- Handle sjson.SetBytes errors in sanitize.go instead of silently swallowing
- Document reverse-engineered magic values in billing.go
- Unexport Credential.CooldownUntil (internal state)
- Replace hardcoded auth bypass paths with map in server.go

internal/embedded/log.go

@@ -0,0 +1,20 @@
+package embedded
+
+import "github.com/rs/zerolog/log"
+
+// logWriter bridges subprocess stdout/stderr to zerolog.
+type logWriter struct {
+	level     string
+	component string
+}
+
+func (w *logWriter) Write(p []byte) (n int, err error) {
+	msg := string(p)
+	switch w.level {
+	case "error":
+		log.Error().Str("component", w.component).Msg(msg)
+	default:
+		log.Debug().Str("component", w.component).Msg(msg)
+	}
+	return len(p), nil
+}