perf(coderd): batch chat heartbeat queries into single UPDATE per interval (#24037)

## Summary

Replaces N per-chat heartbeat goroutines with a single centralized
heartbeat loop that issues one `UPDATE` per 30s interval for all running
chats on a worker.

## Problem

Each running chat spawned a dedicated goroutine that issued an
individual `UPDATE chats SET heartbeat_at = NOW() WHERE id = $1 AND
worker_id = $2 AND status = 'running'` query every 30 seconds. At 10,000
concurrent chats this produces **~333 DB queries/second** just for
heartbeats, plus ~333 `ActivityBumpWorkspace` CTE queries/second from
`trackWorkspaceUsage`.

## Solution

New `UpdateChatHeartbeats` (plural) SQL query replaces the old singular
`UpdateChatHeartbeat`:

```sql
UPDATE chats
SET    heartbeat_at = @now::timestamptz
WHERE  worker_id = @worker_id::uuid
  AND  status = 'running'::chat_status
RETURNING id;
```

A single `heartbeatLoop` goroutine on the `Server`:
1. Ticks every `chatHeartbeatInterval` (30s)
2. Issues one batch UPDATE for all registered chats
3. Detects stolen/completed chats via set-difference (equivalent of old
`rows == 0`)
4. Calls `trackWorkspaceUsage` for surviving chats

`processChat` registers an entry in the heartbeat registry instead of
spawning a goroutine.

## Impact

| Metric | Before (10K chats) | After (10K chats) |
|---|---|---|
| Heartbeat queries/sec | ~333 | ~0.03 (1 per 30s per replica) |
| Heartbeat goroutines | 10,000 | 1 |
| Self-interrupt detection | Per-chat `rows==0` | Batch set-difference |

---

> 🤖 Generated by Coder Agents

<details><summary>Implementation notes</summary>

- Uses `@now` parameter instead of `NOW()` so tests with `quartz.Mock`
can control timestamps.
- `heartbeatEntry` stores `context.CancelCauseFunc` + workspace state
for the centralized loop.
- `recoverStaleChats` is unaffected — it reads `heartbeat_at` which is
still updated.
- The old singular `UpdateChatHeartbeat` is removed entirely.
- `dbauthz` wrapper uses system-level `rbac.ResourceChat` authorization
(same pattern as `AcquireChats`).

</details>

coderd/database/queries/chats.sql

@@ -674,17 +674,20 @@ WHERE
     status = 'running'::chat_status
     AND heartbeat_at < @stale_threshold::timestamptz;
 
--- name: UpdateChatHeartbeat :execrows
--- Bumps the heartbeat timestamp for a running chat so that other
--- replicas know the worker is still alive.
+-- name: UpdateChatHeartbeats :many
+-- Bumps the heartbeat timestamp for the given set of chat IDs,
+-- provided they are still running and owned by the specified
+-- worker. Returns the IDs that were actually updated so the
+-- caller can detect stolen or completed chats via set-difference.
 UPDATE
     chats
 SET
-    heartbeat_at = NOW()
+    heartbeat_at = @now::timestamptz
 WHERE
-    id = @id::uuid
+    id = ANY(@ids::uuid[])
     AND worker_id = @worker_id::uuid
-    AND status = 'running'::chat_status;
+    AND status = 'running'::chat_status
+RETURNING id;
 
 -- name: GetChatDiffStatusByChatID :one
 SELECT