feat: Add allowlist lookup endpoint for single address

[lajay-faith]

Closes #441

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Summary

Adds a lightweight lookup endpoint to check allowlist membership for a single address. This eliminates the need for clients to fetch the full allowlist and search client-side.

Changes

• Added GET /api/allowlist/:address endpoint
• Validates address is a valid Stellar Ed25519 public key
• Returns { address, allowed: true|false } format
• Returns 200 status regardless of membership (not 404 for non-members)
• No authentication required (read-only public query)
• Cleaned up duplicate code in allowlist.ts

Implementation Details

• Queries the full allowlist via contract and checks for address membership
• Uses StellarSdk.StrKey.decodeEd25519PublicKey() for validation
• Returns 400 only for invalid address format
• Returns 200 with allowed: false for valid addresses not in allowlist

Acceptance Criteria

• GET /api/allowlist/:address returns { address, allowed: true|false }
• Validates the address is a valid Stellar public key
• Returns 200 regardless (not 404 for non-members)
• No auth required (read-only public query)

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