Security: Neynar API key exposed to the client via NEXT_PUBLIC_NEYNAR_API_KEY #751

Problem

The Neynar API key is exposed to the browser. NEXT_PUBLIC_NEYNAR_API_KEY is referenced in client code (src/common/helpers/farcaster.ts:541, as an x-api-key header), so Next.js inlines it into the client JS bundle — anyone can read it from the deployed app and burn our Neynar quota (which is already over its CU limit).

Scope

• Actual leak: src/common/helpers/farcaster.ts:541 (client x-api-key). This is what puts the key in the browser bundle.
• Convention: ~22 app/api/* route handlers also read process.env.NEXT_PUBLIC_NEYNAR_API_KEY. Read server-side they don't leak on their own, but the NEXT_PUBLIC_ naming keeps the value in the public env and invites client use.
• The Supabase edge functions already do this correctly — they use a server-only NEYNAR_API_KEY (Deno env). There is currently no server-only NEYNAR_API_KEY in the Vercel/Next environment.

Pre-existing — every /api route predates the Hypersnap work. Flagged by the review on #734 and scoped out, since fixing it touches ~23 files + an env change + a key rotation.

Remediation

1. Rotate the Neynar API key (the current one has been public in the client bundle).
2. Add the new key to Vercel as a server-only NEYNAR_API_KEY (no NEXT_PUBLIC_ prefix).
3. Migrate all app/api/* routes: process.env.NEXT_PUBLIC_NEYNAR_API_KEY → process.env.NEYNAR_API_KEY.
4. Move the client call at src/common/helpers/farcaster.ts:541 behind a server route, so the key never reaches the browser. This is the one that actually leaks — steps 1–3 don't help until this is done.
5. Remove NEXT_PUBLIC_NEYNAR_API_KEY from all environments once nothing references it.
6. (Optional) Add a build/lint check that fails on NEXT_PUBLIC_*_(API_KEY|SECRET|TOKEN) patterns.

Acceptance criteria

• No NEXT_PUBLIC_NEYNAR_API_KEY references remain in client or server code.
• The Neynar key is absent from the built client bundle (grep .next output).
• All /api routes + the former client call work with the server-only NEYNAR_API_KEY.

Files

• src/common/helpers/farcaster.ts:541 (the leak)
• app/api/**/route.ts (~22 files)
• Vercel env config

Refs #734 (review that surfaced this), #715.