feat: add encrypted off-chain blob storage for large prompts

[Jerry-Tekh]

Implements off-chain blob storage for large encrypted prompt payloads. When the encrypted ciphertext exceeds the on-chain 4096-byte limit, the frontend uploads it to configurable blob storage (HTTP blob store, IPFS, S3, etc.) and stores the returned blob reference on-chain instead. The unlock service resolves blob references back to ciphertext before decryption.

Changes:

• Add frontend blob storage utility (src/lib/stellar/blobStorage.ts) with upload/fetch helpers
• Update CreatePromptForm to auto-upload oversized encrypted payloads to blob storage
• Update unlock service (api/prompts/unlock.ts) to resolve blob references before decryption
• Add server-side in-memory blob store (server/src/services/blobStorage.ts) and routes (server/src/routes/blobRoutes.ts) for development/testing
• Register blob routes in Express server
• Add VITE_BLOB_STORAGE_ENDPOINT and VITE_IPFS_GATEWAY env vars to .env.example

closes #499

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[barry01-hash]

Resolve Conflicts