Add GetBlobByCommitment endpoint (retrieve without height)

[tac0turtle]

Summary

Add a GetBlobByCommitment(namespace, commitment) API method that retrieves a blob without requiring the caller to know the height. This is something celestia-node cannot do today.

Motivation

Rollups with external ordering (not Celestia-native ordering) know the namespace and commitment of their blobs but may not know the DA height. Today they must either:

1. Track height externally
2. Scan heights sequentially until they find the blob

celestia-node has an open feature request for this (celestiaorg/celestia-node#4727) but cannot implement it without a new storage index. Apex's SQLite schema makes this trivial.

Implementation

Schema change

Add an index on (namespace, commitment) or just (commitment):

CREATE INDEX idx_blobs_ns_commitment ON blobs(namespace, commitment);

API method

JSON-RPC:

blob.GetByCommitment(namespace, commitment) -> Blob

gRPC:

rpc GetByCommitment(GetByCommitmentRequest) returns (BlobResponse);

Behavior

• Returns the blob matching the namespace + commitment pair
• If multiple blobs match (resubmission at different heights), return the first (lowest height) or all matches
• Returns standard "not found" error if no match

Why this is a differentiator

This positions apex ahead of celestia-node for rollups that:

• Use external sequencing (sovereign rollups, shared sequencers)
• Submit blobs and track them by commitment hash rather than DA height
• Need a simple "do you have this blob?" check

Related

• Implement Celestia node JSON-RPC compatibility layer #2 — JSON-RPC compatibility layer
• Add gRPC API alongside JSON-RPC #3 — gRPC API
• SQLite storage layer implementation #10 — SQLite storage layer (schema change needed)