fix(coderd): use dbtime.Now() instead of time.Now() in test assertions against DB timestamps (#22685)

`time.Now()` has nanosecond precision while Postgres timestamps are
microsecond precision. When tests compare `time.Now()` against
DB-sourced timestamps using `Before`/`After`/`WithinRange`/etc., there
is a non-zero flake risk from the precision mismatch.

This replaces `time.Now()` with `dbtime.Now()` (which rounds to
microsecond precision) in all test assertions that compare against
database timestamps.

Follows from #22684.

## Changes (11 files)

| File | Changes |
|---|---|
| `coderd/apikey_test.go` | 11 comparisons with `ExpiresAt` |
| `coderd/users_test.go` | 2 comparisons with `ExpiresAt` |
| `coderd/oauth2_test.go` | 1 comparison with `token.Expiry` |
| `coderd/workspaces_test.go` | 2 comparisons with `DormantAt` |
| `coderd/workspaceagents_test.go` | 3 comparisons with
`ConnectedAt`/`DisconnectedAt` |
| `coderd/workspaceapps/db_test.go` | 1 comparison with `token.Expiry` |
| `coderd/provisionerdserver/provisionerdserver_test.go` | 1 comparison
with `key.ExpiresAt` |
| `enterprise/coderd/workspaces_test.go` | 1 comparison with `DormantAt`
|
| `enterprise/coderd/license/license_test.go` | 3 `NotBefore` values |
| `enterprise/coderd/licenses_test.go` | 2 `NotBefore` values |
| `enterprise/coderd/users_test.go` | 3 `Next()` comparisons |

## Not changed (intentionally)

- `scaletest/placebo/run_test.go` — compares wall-clock elapsed time,
not DB timestamps
- `cli/server_test.go`, `coderd/jwtutils/jwt_test.go`,
`enterprise/aibridgeproxyd/aibridgeproxyd_test.go` — TLS cert fields,
not DB-stored
- `coderd/azureidentity/azureidentity_test.go` — Azure cert expiry, not
DB


🤖 Generated by Claude Opus 4.6 but reviewed manually.

enterprise/coderd/workspaces_test.go

@@ -4023,7 +4023,7 @@ func TestWorkspaceLock(t *testing.T) {
 		require.NotNil(t, workspace.DeletingAt)
 		require.NotNil(t, workspace.DormantAt)
 		require.Equal(t, workspace.DormantAt.Add(dormantTTL), *workspace.DeletingAt)
-		require.WithinRange(t, *workspace.DormantAt, time.Now().Add(-time.Second), time.Now())
+		require.WithinRange(t, *workspace.DormantAt, dbtime.Now().Add(-time.Second), dbtime.Now())
 		// Locking a workspace shouldn't update the last_used_at.
 		require.Equal(t, lastUsedAt, workspace.LastUsedAt)