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/dbmock/dbmock.go

@@ -7835,19 +7835,19 @@ func (mr *MockStoreMockRecorder) UpdateChatByID(ctx, arg any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatByID", reflect.TypeOf((*MockStore)(nil).UpdateChatByID), ctx, arg)
 }
 
-// UpdateChatHeartbeat mocks base method.
-func (m *MockStore) UpdateChatHeartbeat(ctx context.Context, arg database.UpdateChatHeartbeatParams) (int64, error) {
+// UpdateChatHeartbeats mocks base method.
+func (m *MockStore) UpdateChatHeartbeats(ctx context.Context, arg database.UpdateChatHeartbeatsParams) ([]uuid.UUID, error) {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "UpdateChatHeartbeat", ctx, arg)
-	ret0, _ := ret[0].(int64)
+	ret := m.ctrl.Call(m, "UpdateChatHeartbeats", ctx, arg)
+	ret0, _ := ret[0].([]uuid.UUID)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }
 
-// UpdateChatHeartbeat indicates an expected call of UpdateChatHeartbeat.
-func (mr *MockStoreMockRecorder) UpdateChatHeartbeat(ctx, arg any) *gomock.Call {
+// UpdateChatHeartbeats indicates an expected call of UpdateChatHeartbeats.
+func (mr *MockStoreMockRecorder) UpdateChatHeartbeats(ctx, arg any) *gomock.Call {
 	mr.mock.ctrl.T.Helper()
-	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatHeartbeat", reflect.TypeOf((*MockStore)(nil).UpdateChatHeartbeat), ctx, arg)
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatHeartbeats", reflect.TypeOf((*MockStore)(nil).UpdateChatHeartbeats), ctx, arg)
 }
 
 // UpdateChatLabelsByID mocks base method.