feat: add download mirrors and drop legacy install scripts

Introduce `--mirror` and `DCRUP_MIRROR` so `dcrup` can fetch prebuilt
binaries from multiple CDNs. Default `auto` tries cf → gh → sf → ya
and uses the first successful download.

- dcrup.sh / dcrup.ps1: mirror chain, CLI flag, env override
- remove install.sh / install.ps1 / install_bsd.sh (superseded by dcrup)
- GitHub Actions: upload releases to Cloudflare R2 next to Yandex S3
- docs: README, FAQ, changelog, installation, env vars, contributing

.github/workflows/release.yml

@@ -1312,4 +1312,18 @@ jobs:
 
           aws s3 sync dist/ "s3://${{ secrets.S3_BUCKET_NAME }}/${TAG}/" \
             --endpoint-url https://storage.yandexcloud.net \
-            --region ru-central1
+            --region ru-central1
+
+      - name: Upload Everything to Cloudflare R2
+        env:
+          AWS_ACCESS_KEY_ID: ${{ secrets.R2_ACCESS_KEY_ID }}
+          AWS_SECRET_ACCESS_KEY: ${{ secrets.R2_SECRET_ACCESS_KEY }}
+          AWS_EC2_METADATA_DISABLED: true
+        shell: bash
+        run: |
+          set -euo pipefail
+          TAG="${{ needs.setup.outputs.tag }}"
+
+          aws s3 sync dist/ "s3://${{ secrets.R2_BUCKET_NAME }}/${TAG}/" \
+            --endpoint-url https://${{ secrets.R2_ACCOUNT_ID }}.r2.cloudflarestorage.com \
+            --region auto