Add wallet holdings endpoint returning all creator keys a fan owns

[Chucks1093]

Summary

Fans have no way to see everything they own in one place. To check their portfolio they would have to query each creator individually, which is not practical. A dedicated holdings endpoint that takes a wallet address and returns all creator keys that wallet holds — with quantities and current values — is the foundation of any portfolio view on the client.

Scope

• Add GET /wallets/:address/holdings endpoint
• Response returns an array of entries: { creator_id, creator_handle, key_balance, current_price, total_value }
• Data is read from the key ownership read model maintained by the indexer (not a live on-chain call)
• Wallet address is validated as a well-formed Stellar address before querying
• Entries with a balance of zero are excluded from the response
• Support pagination for wallets with large holdings

Acceptance Criteria

• Endpoint returns all creators a wallet holds keys in, with correct balances
• Zero-balance entries are excluded
• total_value is calculated as key_balance × current_price from the price snapshot
• Returns 400 for a malformed Stellar address
• Returns an empty array (not 404) for a valid address with no holdings
• Integration test covers: wallet with multiple holdings, wallet with no holdings, invalid address

---

Coordinate on Telegram