fix(deploy-preview): reduce channel TTL and cap total preview channels

Reduce expires from 30d to 7d and add totalPreviewChannelLimit: 25 to
prevent hitting Firebase's 50-channel quota limit during bursts of
Dependabot PRs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sserrata

.github/workflows/deploy-preview.yml

@@ -176,8 +176,9 @@ jobs:
           repoToken: "${{ secrets.GITHUB_TOKEN }}"
           firebaseServiceAccount: "${{ steps.creds.outputs.sa_key }}"
           projectId: pandev
-          expires: 30d
+          expires: 7d
           channelId: "pr${{ github.event.number }}"
+          totalPreviewChannelLimit: 25
         env:
           FIREBASE_CLI_PREVIEWS: hostingchannels