[CT-30] Build DocumentMetadataEndpoint

[mftee]

Problem

After a document is submitted there is no endpoint to retrieve the stored submission metadata (submitter identity, document_id, submission timestamp, Stellar transaction hash). Callers who need this information must re-query Stellar directly.

Proposed Solution

Create a DocumentMetadataEndpoint inside contract/module/metadata/ that retrieves and returns the stored submission metadata from Redis.

Acceptance Criteria

• GET /module/metadata/:hash looks up the Redis key submit:{hash} and returns the stored record
• Response contains: document_hash, document_id, submitter, tx_hash, and anchored_at (ISO 8601 formatted timestamp)
• Returns 404 with a document not found error (using the ApiError format from CT-29) when no submission record exists for the given hash
• Validates the hash format using HashValidator::validate_sha256 and returns 400 for invalid input
• Response is served from Redis cache only — does not query Stellar Horizon
• Unit tests cover: hash found, hash not found, invalid hash format, Redis connection error
• Implementation files live inside contract/module/metadata/