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/querier.go

@@ -870,9 +870,11 @@ type sqlcQuerier interface {
 	UpdateAPIKeyByID(ctx context.Context, arg UpdateAPIKeyByIDParams) error
 	UpdateChatBuildAgentBinding(ctx context.Context, arg UpdateChatBuildAgentBindingParams) (Chat, error)
 	UpdateChatByID(ctx context.Context, arg UpdateChatByIDParams) (Chat, error)
-	// Bumps the heartbeat timestamp for a running chat so that other
-	// replicas know the worker is still alive.
-	UpdateChatHeartbeat(ctx context.Context, arg UpdateChatHeartbeatParams) (int64, error)
+	// 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.
+	UpdateChatHeartbeats(ctx context.Context, arg UpdateChatHeartbeatsParams) ([]uuid.UUID, error)
 	UpdateChatLabelsByID(ctx context.Context, arg UpdateChatLabelsByIDParams) (Chat, error)
 	// Updates the cached injected context parts (AGENTS.md +
 	// skills) on the chat row. Called only when context changes