Add /chat/pin endpoint to pin and unpin messages

[rona-sh]

Adds an authenticated POST /chat/pin endpoint that pins and unpins an existing message in individual chats and groups, mapping to whatsmeow's PinInChatMessage.
This PR also bumps go.mau.fi/whatsmeow to latest as it is required for correct PinInChatMessage send behavior (older versions send the stanza without the edit attribute and fail silently).

[gemini-code-assist]

Code Review

This pull request introduces a new /chat/pin endpoint to pin or unpin messages in individual or group chats, complete with documentation in API.md, route registration, handler implementation in handlers.go, and integration tests in pin_test.go. The reviewer recommended supporting the me: prefix convention on message IDs to allow users to pin or unpin their own messages, utilizing the idiomatic !senderJID.IsEmpty() check, and updating the response payload, logging, and documentation to reflect this prefix change.

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[gemini-code-assist]

To allow users to pin or unpin their own messages, we should support the me: prefix convention on the message ID (similar to how it is handled in the React endpoint). Additionally, we should use the idiomatic !senderJID.IsEmpty() check instead of senderJID.String() != "".

		fromMe := false
		msgID := t.Id
		if strings.HasPrefix(msgID, "me:") {
			fromMe = true
			msgID = msgID[len("me:"):]
		}

		key := &waCommon.MessageKey{
			RemoteJID: proto.String(chatJID.String()),
			FromMe:    proto.Bool(fromMe),
			ID:        proto.String(msgID),
		}
		if !senderJID.IsEmpty() {
			key.Participant = proto.String(senderJID.String())
		}

[gemini-code-assist]

Update the response payload and log messages to use the stripped msgID instead of t.Id to ensure consistency with other endpoints (like React).

Suggested change

	response := map[string]interface{}{
	"Chat": chatJID.String(),
	"Id": t.Id,
	"Timestamp": resp.Timestamp.UTC().Format(time.RFC3339),
	}
	if pin {
	response["Details"] = "Message pinned"
	response["DurationSeconds"] = duration
	log.Info().Str("id", t.Id).Str("chat", chatJID.String()).Uint32("duration", duration).Msg("Message pinned")
	} else {
	response["Details"] = "Message unpinned"
	log.Info().Str("id", t.Id).Str("chat", chatJID.String()).Msg("Message unpinned")
	}
	response := map[string]interface{}{
	"Chat": chatJID.String(),
	"Id": msgID,
	"Timestamp": resp.Timestamp.UTC().Format(time.RFC3339),
	}
	if pin {
	response["Details"] = "Message pinned"
	response["DurationSeconds"] = duration
	log.Info().Str("id", msgID).Str("chat", chatJID.String()).Uint32("duration", duration).Msg("Message pinned")
	} else {
	response["Details"] = "Message unpinned"
	log.Info().Str("id", msgID).Str("chat", chatJID.String()).Msg("Message unpinned")
	}

[gemini-code-assist]

To document the ability to pin/unpin your own messages using the me: prefix, we should update the description of the Id field.

Suggested change

	| `Id` | Yes | string | WhatsApp message ID of the message to pin or unpin. |
	| `Id` | Yes | string | WhatsApp message ID of the message to pin or unpin. Prefix with `me:` to pin/unpin your own messages (e.g., `me:3EB0ABCDEF1234567890`). |