Add generic BlobPersistence plugin for Azure/GCS support

[PabloSzx]

Summary

• Adds a new BlobPersistence plugin (src/plugins/blob.js) that accepts a simple adapter interface (put/get/del/init?), allowing callers to bring their own cloud SDK (Azure Blob Storage, GCS, etc.) without adding new dependencies to yhub
• Adds ./plugins/blob package export
• Fixes S3 types export path typo (dist/src/storage/s3.d.ts → dist/src/plugins/s3.d.ts)
• Adds Azure/GCS env var guidance to .env.template

Motivation

We need to deploy yhub to environments with Azure Blob Storage and GCS natively, where S3-compatible storage is not available. The existing plugin architecture already supports this — BlobPersistence is a thin adapter that implements PersistencePlugin and delegates all cloud-specific logic (auth, retries, 404 handling) to the caller-provided adapter.

No changes to existing files (s3.js, persistence.js, types.js, index.js) — S3PersistenceV1 remains unchanged.

Usage

import { BlobPersistence } from '@y/hub/plugins/blob'

const plugin = new BlobPersistence('AzureBlob:v1', {
  init: () => container.createIfNotExists(),
  put: (path, data) => container.getBlockBlobClient(path).upload(data, data.length),
  get: async (path) => { /* ... return Buffer or null */ },
  del: (path) => container.getBlockBlobClient(path).deleteIfExists()
})

createYHub({ persistence: [plugin], ... })

Test plan

• npm run lint passes (standard + tsc)
• Existing tests still pass (no changes to S3 plugin or core)
• Smoke test: instantiate BlobPersistence with an in-memory adapter, verify store/retrieve/delete

[dmonad]

Thanks for the PR @PabloSzx

This wrapper makes it slightly more convenient to write a custom blob storage adapter. The implementation looks solid. However, the overall design looks very similar to the existing plugin interface for custom storage adapters.

I'd rather like to maintain a specific Azure blob adapter. I'm sure there will be a bunch of edge-cases to catch - similarly to the s3 adapter.

At this point I don't want to maintain another abstraction - sorry!