feat(metrics): new-clients KPI and scheduled reminder dispatch

The dashboard promised a new-clients card the API never produced; the
metric now counts distinct customer emails whose earliest-ever booking
lands in the window, joining the other four KPIs with the same trend
comparison. The card grid rebalances for five items.

Reminder dispatch also had no trigger — the endpoint existed but
nothing called it. A scheduled GitHub Actions workflow now posts to it
every 15 minutes using the job-runner key, so reminders actually fire
once the two secrets are configured.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/workflows/reminder-dispatch.yml

@@ -0,0 +1,32 @@
+name: Reminder Dispatch
+
+on:
+  schedule:
+    # Every 15 minutes. GitHub scheduled workflows may run a few minutes late;
+    # reminder jobs are timestamp-based so lateness only delays, never skips.
+    - cron: "*/15 * * * *"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  dispatch:
+    name: Dispatch due reminders
+    runs-on: ubuntu-latest
+    steps:
+      - name: Call dispatch endpoint
+        env:
+          API_URL: ${{ secrets.BOOKRA_API_URL }}
+          JOB_KEY: ${{ secrets.BOOKRA_JOB_RUNNER_KEY }}
+        run: |
+          if [ -z "$API_URL" ] || [ -z "$JOB_KEY" ]; then
+            echo "BOOKRA_API_URL / BOOKRA_JOB_RUNNER_KEY secrets not configured — skipping."
+            exit 0
+          fi
+          status=$(curl -sS -o /tmp/dispatch.json -w "%{http_code}" \
+            -X POST "$API_URL/v1/internal/jobs/reminders/dispatch" \
+            -H "Content-Type: application/json" \
+            -H "X-Bookra-Job-Key: $JOB_KEY" \
+            -d '{"limit": 100}')
+          cat /tmp/dispatch.json
+          test "$status" = "200"