fix(api): prevent background task garbage collection in lifespan startup

[alexzhu0]

Closes #6552

Problem

Two asyncio.create_task(...) calls in keep/api/api.py create background tasks without holding a strong reference to the returned Task object. The Python asyncio docs warn that the event loop only keeps a weak reference to a task, so a fire-and-forget task can be garbage-collected mid-execution and silently cancelled — no traceback, just missing work.

Important: Save a reference to the result of [create_task()], to avoid a task disappearing mid-execution. The event loop only keeps weak references to tasks. A task that isn't referenced elsewhere may be garbage collected at any time, even before it's done.
— https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task

Affected sites

1. startup() line 183 — the maintenance-window watcher:

   asyncio.create_task(process_watcher_task.async_process_watcher())

   Zero strong references. The Task can be collected before it ever runs async_process_watcher, and when it does the watcher just disappears.

2. lifespan() line 234 — the debug pending-task logger:

   asyncio.create_task(check_pending_tasks(background_tasks))

   check_pending_tasks is passed the background_tasks set as an argument, but the Task wrapping the coroutine is never added to that set. So this task also lacks a strong reference and is GC-eligible.

Root cause

CPython garbage-collects coroutine Tasks the same way it collects any other object. The asyncio event loop only keeps weak references to running tasks (asyncio.all_tasks() is implemented via a WeakSet). If no other code holds a strong reference, the GC is free to reclaim the Task at any GC pass, which cancels it.

Changes

keep/api/api.py — one file, +10 / -5 lines:

• startup() now accepts an optional background_tasks: set | None = None parameter; when present the watcher task is added to it and registers a done-callback to discard itself on completion (task.add_done_callback(background_tasks.discard)).
• lifespan() passes its existing background_tasks set into startup(), and now also adds the debug check_pending_tasks task to that same set with the same done-callback. The set was already being created here for request-context use; this just makes the lifespan tasks share it.
• The set type annotation is tightened to set[asyncio.Task] for clarity.

Backward compatible: any other caller of startup() continues to work because the new parameter defaults to None.

Testing

Manual reasoning + asyncio docs reference; the GC-cancellation behaviour is established Python asyncio semantics and the fix matches the pattern recommended in the stdlib docs (hold strong reference in a set, discard via done-callback).

I considered adding a unit test but a test that reliably triggers gc.collect() mid-task in pytest would be fragile and the bug class is well-documented. Happy to add a regression test that simply asserts the watcher Task ends up in app.state if maintainers prefer.

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