[T2.3] Build lib/airdrop/twitterapi.ts (lookupXUser)

[realproject7]

Epic: #1229 · Spec: §3.2, §6.3 · Type: CODE · Estimate: 0.3 day · Depends on: T0.1

Scope

Create lib/airdrop/twitterapi.ts exporting:

```ts
export async function lookupXUser(username: string): Promise<{
display_name: string;
avatar_url: string;
follower_count: number;
x_user_id: string;
bio_snippet: string;
} | null>
```

• Single twitterapi.io call: GET /v2/users/by/username/{username}
• Cache: 5-min in-memory LRU keyed by username (covers retries/debouncing only, not cross-user dedup — cost is $0.0003/call, marginal gain)
• Handle 404 → return null
• Handle 5xx / network errors → throw (callers handle R16 graceful degradation)
• Use process.env.TWITTERAPI_IO_KEY

Acceptance

• Successful lookup returns shape above
• 404 returns null
• 5xx throws
• LRU cache hit on second call within 5 min (verify via test)
• No call made if cache hit

Dependencies

T0.1 (API key in env)

[realproject7]

Completed by PR #1276.