feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/templates/pages/home.html

@@ -15,7 +15,7 @@
         </span>
         <span class="drop-title">Drop files to upload</span>
         <span class="drop-copy">or click to browse</span>
-        <span class="drop-meta">{{if .Data.IsAdmin}}Admin upload: no file size limit{{else}}Max file size: {{.Data.MaxUploadSize}}{{end}} · Links expire in 7 days</span>
+        <span class="drop-meta">Max file size: {{.Data.MaxUploadSize}} · {{.Data.LimitSummary}}</span>
         <input id="file-input" name="file" type="file" multiple>
       </label>