fix(security): fail fast on a missing/placeholder/public LYRA_SECRET_KEY

.env.example shipped a valid, PUBLIC base64 key — if a user didn't replace it,
every credential got encrypted with a publicly-known key and everything still
"worked," so the footgun was silent.

- .env.example now ships an obvious placeholder (CHANGE_ME_generate_with_...)
  with a "you MUST replace this" note and the openssl hint.
- web + worker validate LYRA_SECRET_KEY at startup: missing / placeholder /
  wrong-length ⇒ loud FATAL banner and refuse to start (web via the Next.js
  instrumentation hook, worker via _require_secret_key before touching the DB).
  The old public example key ⇒ a loud WARNING but not fatal, since an existing
  install may have encrypted its creds under it (rotating needs re-entry).
- Validation lives in an edge-safe web/src/lib/secret-key.ts (no node:crypto)
  so the instrumentation hook bundles for both runtimes; crypto.ts re-exports.
- README emphasizes generating a fresh key + keeping it stable.

Verified live: worker exits 1 on placeholder/unset before any DB call; web
instrumentation throws and refuses to serve on the placeholder. web 138 tests,
worker 204/7-skip, tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/crypto.ts

@@ -1,5 +1,9 @@
 import { createCipheriv, createDecipheriv, randomBytes } from "node:crypto";
 
+// Re-exported for existing callers; the checks live in an edge-safe module (secret-key.ts)
+// so the instrumentation hook can import them without pulling in node:crypto.
+export { secretKeyProblem, PLACEHOLDER_SECRET_KEY } from "./secret-key";
+
 function key(): Buffer {
   const b64 = process.env.LYRA_SECRET_KEY;
   if (!b64) throw new Error("LYRA_SECRET_KEY is not set");