Add X-Persist header to control client-side local storage

Introduces a new X-Persist (or Persist) publish parameter. When set to
"no", the server includes "persist": false in the broadcast JSON payload,
signalling clients that they should not save the message to local history.

This is intentionally separate from X-Cache (server-side caching) so that
publishers can control client persistence independently — e.g. ephemeral
clipboard sync messages that should be delivered but not stored.

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

model/model.go

@@ -47,6 +47,7 @@ type Message struct {
 	PollID      string      `json:"poll_id,omitempty"`
 	ContentType string      `json:"content_type,omitempty"` // text/plain by default (if empty), or text/markdown
 	Encoding    string      `json:"encoding,omitempty"`     // Empty for raw UTF-8, or "base64" for encoded bytes
+	Persist     *bool       `json:"persist,omitempty"`      // If false, clients should not save the message to local history
 	Sender      netip.Addr  `json:"-"`                      // IP address of uploader, used for rate limiting
 	User        string      `json:"-"`                      // UserID of the uploader, used to associated attachments
 }

server/server.go

@@ -1166,6 +1166,7 @@ func (s *Server) parsePublishParams(r *http.Request, m *model.Message) (cache bo
 		}
 	}
 	cache = readBoolParam(r, true, "x-cache", "cache")
+	m.Persist = readOptionalBoolParam(r, "x-persist", "persist")
 	firebase = readBoolParam(r, true, "x-firebase", "firebase")
 	m.Title = readParam(r, "x-title", "title", "t")
 	m.Click = readParam(r, "x-click", "click")

server/util.go

@@ -30,6 +30,16 @@ var (
 	forwardedHeaderRegex = regexp.MustCompile(`(?i)\bfor="?(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}|\[[0-9a-f:]+])(?::\d+)?"?`)
 )
 
+// readOptionalBoolParam returns nil if the param is not set, otherwise a pointer to the parsed bool value.
+func readOptionalBoolParam(r *http.Request, names ...string) *bool {
+	value := strings.ToLower(readParam(r, names...))
+	if value == "" || !isBoolValue(value) {
+		return nil
+	}
+	b := toBool(value)
+	return &b
+}
+
 func readBoolParam(r *http.Request, defaultValue bool, names ...string) bool {
 	value := strings.ToLower(readParam(r, names...))
 	if value == "" {