[Story] Threads: Subscribing to a Thread

[manuroe]

Description

• As a user
• I want to subscribe to a thread
• So that I receive notifications for its activity even if I haven't participated yet.

Acceptance criteria

• A "Subscribe" option is available in the thread menu.
• Subscribed threads send notifications for new messages, mentions, and replies.
• Subscriptions are synced across platforms.

Dependencies

• [Prep] Threads: Design a participation model API in Matrix #2777

Out of scope

• Nothing

Questions

• [ ]

Subtasks

Android

iOS

Rust

Other

[reivilibre]

Backend work on this will be closed by:

• Add experimental support for MSC4308: Thread Subscriptions extension to Sliding Sync when MSC4306 and MSC4186 are enabled. synapse#18695 (likely nearing end of its review)
• MSC4308: Thread Subscriptions extension to Sliding Sync tests  matrix-org/complement#802