feat(water-log): run Smartsheet sync via Gitea Actions cron + persist secrets on deploy

- New .gitea/workflows/smartsheet-cron.yml: hits POST /api/water-log/smartsheet-sync
  every 15min with SMARTSHEET_CRON_SECRET bearer. Replaces the no-op
  vercel.json cron entry (ignored on self-hosted Gitea).
- deploy.yml now writes SMARTSHEET_TOKEN_ENC_KEY and SMARTSHEET_CRON_SECRET
  to the runtime .env on every deploy so we don't silently lose them.
- Dropped the unused cron entry from vercel.json.

.gitea/workflows/smartsheet-cron.yml

@@ -0,0 +1,40 @@
+name: Smartsheet sync cron
+
+# Replaces the `vercel.json` cron entry (ignored on self-hosted Gitea).
+# Drains the Smartsheet sync queue every 15 minutes via the running
+# Next.js app's POST /api/water-log/smartsheet-sync endpoint.
+
+on:
+  schedule:
+    # Every 15 minutes. Gitea Actions uses standard 5-field cron syntax;
+    # `0,15,30,45 * * * *` is the explicit form to avoid `*/15`
+    # parser quirks across older Gitea versions.
+    - cron: "0,15,30,45 * * * *"
+  workflow_dispatch:
+
+jobs:
+  drain-sync-queue:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Hit /api/water-log/smartsheet-sync
+        env:
+          SMARTSHEET_CRON_SECRET: ${{ secrets.SMARTSHEET_CRON_SECRET }}
+          SITE_URL: ${{ secrets.NEXT_PUBLIC_SITE_URL }}
+        run: |
+          set -euo pipefail
+          if [ -z "$SMARTSHEET_CRON_SECRET" ]; then
+            echo "SMARTSHEET_CRON_SECRET secret not set — skipping."
+            exit 0
+          fi
+          if [ -z "$SITE_URL" ]; then
+            echo "NEXT_PUBLIC_SITE_URL secret not set — skipping."
+            exit 0
+          fi
+          echo "POST $SITE_URL/api/water-log/smartsheet-sync"
+          curl -fsS --max-time 60 -X POST \
+            -H "Authorization: Bearer $SMARTSHEET_CRON_SECRET" \
+            -H "Content-Type: application/json" \
+            -H "User-Agent: gitea-smartsheet-cron/1" \
+            "${SITE_URL%/}/api/water-log/smartsheet-sync" \
+            --data '{}' \
+            -w "\n[http %{http_code} in %{time_total}s]\n"