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>

.env.example

@@ -3,8 +3,10 @@ POSTGRES_PASSWORD=lyra
 POSTGRES_DB=lyra
 DATABASE_URL=postgresql://lyra:lyra@db:5432/lyra
 
-# 32 random bytes, base64. Generate with: openssl rand -base64 32
-LYRA_SECRET_KEY=MDEyMzQ1Njc4OWFiY2RlZjAxMjM0NTY3ODlhYmNkZWY=
+# Encryption key for stored credentials — 32 random bytes, base64. YOU MUST REPLACE THIS.
+# Generate your own:  openssl rand -base64 32
+# Leaving the placeholder (or an empty key) makes web + worker refuse to start.
+LYRA_SECRET_KEY=CHANGE_ME_generate_with_openssl_rand_base64_32
 
 # Shared password protecting the whole web UI + API. When set, every request must sign in
 # (one shared password, no per-user accounts). LEAVE UNSET AND THE APP IS OPEN to anyone