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>

worker/tests/test_secret_key.py

@@ -0,0 +1,40 @@
+import base64
+
+import pytest
+
+from lyra_worker.crypto import PLACEHOLDER_SECRET_KEY, secret_key_problem
+
+_GOOD = base64.b64encode(b"x" * 32).decode()
+_LEGACY = "MDEyMzQ1Njc4OWFiY2RlZjAxMjM0NTY3ODlhYmNkZWY="
+
+
+@pytest.fixture(autouse=True)
+def _clean_env(monkeypatch):
+    monkeypatch.delenv("LYRA_SECRET_KEY", raising=False)
+
+
+def test_missing_key_is_fatal(monkeypatch):
+    assert secret_key_problem() == ("LYRA_SECRET_KEY is not set", True)
+
+
+def test_placeholder_is_fatal(monkeypatch):
+    monkeypatch.setenv("LYRA_SECRET_KEY", PLACEHOLDER_SECRET_KEY)
+    msg, fatal = secret_key_problem()
+    assert fatal is True and "placeholder" in msg
+
+
+def test_wrong_length_is_fatal(monkeypatch):
+    monkeypatch.setenv("LYRA_SECRET_KEY", base64.b64encode(b"short").decode())
+    msg, fatal = secret_key_problem()
+    assert fatal is True and "32 bytes" in msg
+
+
+def test_legacy_public_key_warns_not_fatal(monkeypatch):
+    monkeypatch.setenv("LYRA_SECRET_KEY", _LEGACY)
+    msg, fatal = secret_key_problem()
+    assert fatal is False and "PUBLIC" in msg
+
+
+def test_good_key_is_fine(monkeypatch):
+    monkeypatch.setenv("LYRA_SECRET_KEY", _GOOD)
+    assert secret_key_problem() is None