Fix Farcaster identity: use Steemhunt API (free) with Neynar fallback

[realproject7]

Problem

Farcaster identity display (P7-4, issue #39) is effectively broken — it depends on NEYNAR_API_KEY which is not configured. All wallet addresses show as truncated hex (0x0175...4dBf) instead of Farcaster usernames + avatars.

The current implementation in lib/farcaster.ts uses Neynar API only and silently returns null when the API key is missing (line 39).

Solution

Port the Steemhunt-first, Neynar-fallback pattern from the dropcast repo. Steemhunt's Farcaster Indexer API (https://fc.hunt.town) is free and requires no API key.

Reference implementation (dropcast)

• dropcast/lib/farcaster-indexer.ts — Steemhunt client with:
  • getUserByWallet(address) — primary lookup
  • getUserByFid(fid) — FID-based lookup
  • 1-hour cache TTL, in-flight deduplication, circuit breaker, retry with backoff
  • Negative caching for known failures (422 → 4h, other 4xx → 1h)
• dropcast/lib/neynar.ts — Neynar fallback with getUserByAddress(address)

Steemhunt API

• Base URL: https://fc.hunt.town
• Endpoint: GET /users/byWallet/{address} — returns { fid, username, displayName, pfpUrl, addresses, ... }
• No API key required (free)
• Source: https://github.com/Steemhunt/farcaster-indexer

Required changes

1. Replace lib/farcaster.ts with two-tier lookup

Rewrite lookupByAddress() to:

1. Try Steemhunt https://fc.hunt.town/users/byWallet/{address} first
2. If Steemhunt fails/returns null → fall back to Neynar (if NEYNAR_API_KEY is set)
3. Return FarcasterProfile or null

Simplify from dropcast — PlotLink doesn't need the full circuit breaker/retry/negative-cache complexity. A simple implementation is fine:

• In-memory cache (1h TTL) using a Map (no node-cache dependency needed)
• 3s timeout per request
• Steemhunt first → Neynar fallback

2. Update FarcasterProfile type

Current type is fine. Map Steemhunt response fields:

• fid → fid
• username → username
• displayName → displayName
• pfpUrl → pfpUrl

3. No changes needed to components

WriterIdentity.tsx and WriterIdentityClient.tsx already consume FarcasterProfile via lookupByAddress() — they'll work once the lookup actually returns data.

Files

• lib/farcaster.ts — rewrite lookup logic (Steemhunt primary, Neynar fallback)
• Reference: dropcast/lib/farcaster-indexer.ts (Steemhunt client pattern)

Acceptance criteria

• Farcaster username + avatar displays for wallets linked to Farcaster accounts
• Works without NEYNAR_API_KEY (Steemhunt is free)
• Falls back to Neynar if Steemhunt returns null and API key is configured
• Falls back to truncated address if both fail
• npm run typecheck passes