feat(presence): expose POST /message/subscribe to receive contact presence

[nicolasnovis]

Addresses part of #146 (the SubscribePresence half).

whatsmeow only delivers events.Presence (online/offline/last-seen) for JIDs you've explicitly SubscribePresence'd, and only while the instance is marked available. Today there is no route to subscribe, so the existing events.Presence handler never fires.

This adds POST /message/subscribe {number} which sends available first (idempotent — chat presence and the background presence loop already do this) and then calls client.SubscribePresence(jid). Subscriptions are ephemeral (reset on reconnect), so callers re-subscribe when a chat is opened.

It also enriches the Presence webhook payload with explicit top-level from and lastSeen (unix) fields, so consumers don't have to depend on types.JID / time.Time JSON marshaling.

Tested in production: opening a chat subscribes the contact, and online/offline/last-seen events flow through the webhook.

Summary by Sourcery

Add an endpoint and service support to subscribe to WhatsApp contact presence and enrich emitted presence webhooks.

New Features:

• Expose POST /message/subscribe API to subscribe an instance to a contact's presence updates.
• Introduce a SubscribePresence service struct and method to manage presence subscriptions for specific numbers.
• Extend presence webhook payloads with explicit from and lastSeen fields for easier consumption.

Enhancements:

• Wire the new subscribe presence handler into the routing layer with existing JID/number validation middleware.

[sourcery-ai]

Reviewer's Guide

Adds a new POST /message/subscribe endpoint and service method to explicitly subscribe an instance to a contact’s presence updates in WhatsApp (via whatsmeow), and enriches Presence webhook events with top-level from and lastSeen fields.

Sequence diagram for POST /message/subscribe presence subscription flow

sequenceDiagram
    actor ApiClient
    participant GinRouter
    participant MessageHandler
    participant MessageService
    participant MyClient

    ApiClient->>GinRouter: POST /message/subscribe {number}
    GinRouter->>MessageHandler: SubscribePresence(ctx)
    MessageHandler->>MessageHandler: ctx.MustGet(instance)
    MessageHandler->>MessageHandler: ctx.ShouldBindBodyWithJSON(data)
    MessageHandler->>MessageService: SubscribePresence(data, instance)
    MessageService->>MessageService: ensureClientConnected(instance.Id)
    MessageService->>MessageService: utils.ParseJID(data.Number)
    MessageService->>MessageService: utils.CanonicalJID(recipient)
    MessageService->>MyClient: SendPresence(ctx, types.PresenceAvailable)
    MessageService->>MyClient: SubscribePresence(ctx, recipient)
    MessageService-->>MessageHandler: nil
    MessageHandler-->>ApiClient: 200 {"message":"success"}

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Sequence diagram for updated Presence webhook event enrichment

sequenceDiagram
    participant WhatsApp
    participant MyClient

    WhatsApp-->>MyClient: events.Presence evt
    MyClient->>MyClient: myEventHandler(evt)
    MyClient->>MyClient: postMap["event"] = "Presence"
    MyClient->>MyClient: postMap["from"] = evt.From.String()
    alt evt.Unavailable && !evt.LastSeen.IsZero()
        MyClient->>MyClient: postMap["state"] = "offline"
        MyClient->>MyClient: postMap["lastSeen"] = evt.LastSeen.Unix()
    else evt.Unavailable && evt.LastSeen.IsZero()
        MyClient->>MyClient: postMap["state"] = "offline"
    else !evt.Unavailable
        MyClient->>MyClient: postMap["state"] = "online"
    end

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File-Level Changes

Change	Details	Files
Introduce HTTP handler and routing for subscribing to contact presence.	Extend MessageHandler interface with a SubscribePresence method. Implement SubscribePresence handler that resolves the instance, validates request body (number required), delegates to messageService.SubscribePresence, and returns appropriate HTTP status codes. Register POST /message/subscribe route under /message with existing JID validation middleware.	pkg/message/handler/message_handler.go pkg/routes/routes.go
Add service-layer support for presence subscription using whatsmeow client.	Extend MessageService interface with SubscribePresence method and define SubscribePresenceStruct with a Number field. Implement SubscribePresence to ensure a connected client, parse and canonicalize the target JID from the phone number, send PresenceAvailable (non-fatal if it fails), invoke client.SubscribePresence, and log success or errors. Ensure errors for invalid phone numbers and subscribe failures are surfaced to the handler.	pkg/message/service/message_service.go
Enrich Presence webhook payloads with explicit metadata to simplify consumers.	In the whatsmeow event handler, for events.Presence, add top-level from (JID string) to the webhook payload. When the presence is unavailable and LastSeen is non-zero, add a top-level lastSeen field as a Unix timestamp while preserving existing logging behavior.	pkg/whatsmeow/service/whatsmeow.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• In SubscribePresence handler you declare var data *message_service.SubscribePresenceStruct and pass &data to ShouldBindBodyWithJSON; this creates a pointer-to-pointer pattern that is unusual for Gin and inconsistent with the other handlers—consider using a value struct (e.g. var data message_service.SubscribePresenceStruct and binding into &data) for simpler and safer binding.
• In MessageService.SubscribePresence you use context.Background() for SendPresence and SubscribePresence; consider threading through a request-scoped context (or a context with timeout) so these calls respect client cancellation and avoid potentially hanging operations.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `SubscribePresence` handler you declare `var data *message_service.SubscribePresenceStruct` and pass `&data` to `ShouldBindBodyWithJSON`; this creates a pointer-to-pointer pattern that is unusual for Gin and inconsistent with the other handlers—consider using a value struct (e.g. `var data message_service.SubscribePresenceStruct` and binding into `&data`) for simpler and safer binding.
- In `MessageService.SubscribePresence` you use `context.Background()` for `SendPresence` and `SubscribePresence`; consider threading through a request-scoped context (or a context with timeout) so these calls respect client cancellation and avoid potentially hanging operations.

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