fix(ai): keep a chat turn's Durable Object alive with a heartbeat alarm

[Makisuo]

Stack 6 of 6, draft: not yet verified end to end. Based on #903.

Why

A plain fetch() never keeps a Durable Object in memory, even while the response body streams, and an object with no incoming request or event for 70–140 seconds is evicted (Cloudflare docs). A chat turn survives because the open page holds a subscription. An autonomous investigation that nobody is watching has nothing holding it. After #898 that is every alert-triggered investigation.

Locally, an investigation started from the hub with its page never opened went silent 2m17s in, right after a batch of tool results. It raised no error, the two-minute stream idle timeout from #903 never fired, and there was no close-out. It sat until the 15-minute stale sweep marked it failed. A timer that never fires suggests the fiber itself was gone, which fits eviction.

What changes

• While a turn holds the slot, the object re-arms its alarm every 30 seconds. The alarm is the event that prevents eviction.
• If the alarm lands on an activation that holds the claim but is not running the turn, the object was evicted anyway (a deploy). It then releases the slot with a terminal event instead of waiting for the watchdog.

Not verified

The confirming run did not complete. The first attempt was invalid because closing the Browser pane stopped the dev stack underneath the run. Two restarts then hung before planning, while the local Docker Postgres stopped answering queries. Before merging, run one unobserved investigation on a preview or locally and confirm it reaches diagnosed or inconclusive, and confirm alarms fire under alchemy dev.

Unit-tested against the real SQLite fake: the alarm is armed on beginTurn, re-armed while the turn runs, not re-armed after it ends, and an orphaned slot is released on a fresh activation. Typechecks pass on apps/ai, apps/api and apps/alerting.

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