polish: improve step names and logging clarity

- Rename step from 'Get dates for last week' to 'Get dates for last 14 days'
- Add consistent warning messages when metrics files are missing
- Use console.warn for missing file scenarios with [parse-metrics] prefix
- Improve debugging visibility for fallback behavior

.github/workflows/weekly-metrics-discord.yml

@@ -25,7 +25,7 @@ jobs:
         with:
           node-version: '20'
 
-      - name: Get dates for last week
+      - name: Get dates for last 14 days
         run: |
           set -Eeuo pipefail
           # Last 14 days