Feature: S3-Compatible Object Storage API

## Summary

Add an S3-compatible API layer on top of Apex so Celestia blobs can be accessed as objects in buckets.

This would allow developers to interact with Celestia-backed storage using standard S3 tooling such as:

- AWS CLI
- S3 SDKs
- MinIO clients

Example usage:

```
aws s3 cp file.txt s3://celestia-bucket/file.txt
```

Under the hood Apex would:

1. Chunk the object into blobs
2. Submit blob transactions to Celestia
3. Store object metadata in SQLite
4. Reconstruct objects from blobs when requested

## Motivation

Apex already provides:

- JSON RPC interface
- gRPC interface
- SQLite cache/index layer

Adding an S3-compatible API would make Apex usable with a huge ecosystem of existing tooling and SDKs.

## Architecture

```
S3 API
↓
SQLite metadata + cache
↓
Apex blob client
↓
Celestia node
```

Celestia remains the canonical storage layer while SQLite acts as a local index and cache.