Switch from S3 to Backblaze B2 storage

- Update opam-publish script to use b2 CLI via uv
- Update README with B2 setup and Cloudflare CDN instructions
- Fix simdjsont checksum for new B2 upload

README.md

···
 
 ```
 ┌──────────────┐   opam-publish   ┌─────────────┐
-│ Your Package │ ────────────────▶│ S3 Storage  │◀── tarballs
-└──────────────┘                  └─────────────┘
+│ Your Package │ ────────────────▶│ Backblaze   │◀── tarballs
+└──────────────┘                  │     B2      │
+                                  └─────────────┘
                                          │
                                          ▼
                                   ┌─────────────┐
-                                  │ Cloudflare  │◀── public reads
+                                  │ Cloudflare  │◀── CDN + custom domain
                                   └─────────────┘
                                          
 ┌──────────────┐   opam-publish   ┌─────────────┐
···
 
 ### Requirements
 
-- `mc` (MinIO client, for S3-compatible uploads)
+- `uv` (for running b2 CLI tool)
 - `git` (for pushing to this repo)
 - `tar`, `sha256sum` (standard Unix tools)
 - `opam` (optional, for linting validation)
···
 
 ### Configuration
 
-**1. Configure mc alias** (stores credentials securely in `~/.mc/`):
+**1. Authorize b2 CLI**:
 
 ```bash
-mc alias set mys3 https://your-s3-endpoint ACCESS_KEY SECRET_KEY
+uv tool run b2 account authorize <applicationKeyId> <applicationKey>
 ```
+
+You can get your application key from the Backblaze B2 dashboard under "App Keys".
 
 **2. Create opam-publish config** (`~/.config/opam-publish/config`):
 
···
 chmod 700 ~/.config/opam-publish
 
 cat > ~/.config/opam-publish/config << 'EOF'
-MC_ALIAS=mys3
-S3_BUCKET=your-bucket
+B2_BUCKET=your-bucket
 PUBLIC_URL_BASE=https://packages.yourdomain.com
 OPAM_REPO_URL=git@your-git-host:path/to/this/repo
 EOF
···
 1. Validates `.opam` file (required fields, runs `opam lint` if available)
 2. Builds a source tarball (excludes `.git`, `_build`, `_opam`)
 3. Computes SHA256 checksum
-4. Uploads tarball to S3
+4. Uploads tarball to Backblaze B2
 5. Clones this repo, adds package metadata with URL + checksum
 6. Pushes commit to this repo
 
···
     ok
 ==> Building mylib.1.0.0.tbz...
     42K
-==> Uploading to S3...
-    https://packages.example.com/mylib/mylib.1.0.0.tbz
+==> Uploading to B2...
+    https://packages.yourdomain.com/mylib/mylib.1.0.0.tbz
 ==> Cloning opam repository...
 ==> Pushing to opam repository...
 ==> Published mylib 1.0.0
···
    opam-publish
    ```
 
+## Cloudflare CDN Setup (Optional)
+
+To serve packages via a custom domain with Cloudflare:
+
+### 1. DNS Record
+
+Add a CNAME record pointing to your B2 bucket:
+
+```
+Type: CNAME
+Name: opam (or your subdomain)
+Target: f00X.backblazeb2.com  # Replace X with your cluster number from `b2 account get`
+Proxy: ON (orange cloud)
+```
+
+### 2. Transform Rule
+
+Create a URL rewrite rule in **Rules > Transform Rules > Rewrite URL**:
+
+```
+Name: B2 Bucket Rewrite
+When: (http.host eq "packages.yourdomain.com")
+Rewrite to:
+  Path: Dynamic
+  Expression: concat("/file/your-bucket", http.request.uri.path)
+```
+
+### 3. SSL Settings
+
+Set SSL mode to **Full** in **SSL/TLS > Overview**.
+
+### 4. Cache Settings (Recommended)
+
+Create a cache rule for better performance:
+
+```
+URL: packages.yourdomain.com/*
+Cache Level: Cache Everything
+Edge Cache TTL: 1 month
+```
+
 ## Troubleshooting
 
 ### "version not found"
···
 
 Each version can only be published once. Bump the version number.
 
-### S3 upload fails
+### B2 upload fails
 
-Check mc alias and bucket access:
+Check b2 authorization:
 
 ```bash
-mc alias list
-mc ls mys3/your-bucket
+uv tool run b2 account get
+uv tool run b2 bucket list
+```
+
+Re-authorize if needed:
+
+```bash
+uv tool run b2 account authorize <applicationKeyId> <applicationKey>
 ```
 
 ### Git push fails

bin/opam-publish

···
 Create it with:
     mkdir -p ~/.config/opam-publish
     cat > ~/.config/opam-publish/config << 'EOF'
-MC_ALIAS=mys3
-S3_BUCKET=your-bucket
+B2_BUCKET=your-bucket
 PUBLIC_URL_BASE=https://packages.yourdomain.com
 OPAM_REPO_URL=git@your-git-host:your-repo
 EOF
 
-Then configure mc:
-    mc alias set mys3 https://your-s3-endpoint ACCESS_KEY SECRET_KEY"
+Then authorize b2:
+    uv tool run b2 account authorize <applicationKeyId> <applicationKey>"
     source "$CONFIG_FILE"
 }
 
···
     echo "    $(du -h "${archive}" | cut -f1)"
 }
 
-upload_s3() {
+upload_b2() {
     local pkg="$1" archive="$2"
     local filename
     filename=$(basename "${archive}")
-    echo "==> Uploading to S3..."
-    mc cp --quiet "${archive}" "${MC_ALIAS}/${S3_BUCKET}/${pkg}/${filename}"
-    echo "    ${PUBLIC_URL_BASE}/${pkg}/${filename}"
+    local b2_path="${pkg}/${filename}"
+    echo "==> Uploading to B2..."
+    uv tool run b2 file upload --no-progress "${B2_BUCKET}" "${archive}" "${b2_path}"
+    echo "    ${PUBLIC_URL_BASE}/${b2_path}"
 }
 
 publish_opam() {
···
     local checksum
     checksum=$(sha256sum "/tmp/${archive}" | cut -d' ' -f1)
     
-    upload_s3 "$pkg" "/tmp/${archive}"
+    upload_b2 "$pkg" "/tmp/${archive}"
     publish_opam "$pkg" "$version" "$archive" "$checksum"
     
     rm -f "/tmp/${archive}"

packages/simdjsont/simdjsont.0.1.0/opam

···
 
 url {
   src: "https://opam.qswarm.com/simdjsont/simdjsont.0.1.0.tbz"
-  checksum: "sha256=abf633c49ed29642417347d8c457eb642e3e0dae07a4ce2ec6b91c32feaa7334"
+  checksum: "sha256=dbbbebd1401fedba7542684b9a9ccb35f5c34e34a2e01a1480544e024b1bcff3"
 }