[T2.4] Build lib/airdrop/activation-verify.ts (verifyFc)

[realproject7]

Epic: #1229 · Spec: §3.3, §6.3 · Type: CODE · Estimate: 0.3 day

Scope

Create lib/airdrop/activation-verify.ts exporting:

```ts
export async function verifyFc(
username: string,
plotlinkFid: number
): Promise<
| { ok: true; fid: number }
| { ok: false; error: 'user_not_found' | 'not_following' | 'neynar_error' }

```

Wraps the 2 Neynar calls:

1. getUserByUsername(username) → fid
2. checkFollow(fid, plotlinkFid) → boolean

Partial-failure semantics (per §6.3):

• (1) 4xx/not-found → { ok: false, error: 'user_not_found' }
• (1) succeeds, (2) returns false → { ok: false, error: 'not_following' }
• (1) succeeds, (2) returns true → { ok: true, fid }
• Any 5xx / network → { ok: false, error: 'neynar_error' }

Use existing Neynar SDK / API key already used by DropCast.

Acceptance

• User-not-found case returns correct error
• Not-following case returns correct error (no partial DB write)
• Following case returns { ok: true, fid }
• Neynar 5xx surfaced as neynar_error
• No DB writes happen in this lib (caller endpoint handles persistence)

Dependencies

None (Neynar key already configured)

[realproject7]

Completed by PR #1277.