security: close SEC-H4/H6 gaps from second-pass review

- SEC-H6: remove conditional audience check in mcp/index.ts — audience is
  now always enforced against the mcpResource URL. Add migration to revoke
  pre-existing oauth_tokens with audience=NULL so dead rows don't linger.
- SEC-H4: validate doc.issuer against config.issuer inside discover() to
  prevent a MITM'd discovery doc from supplying a crafted expected issuer.
  verifyIdToken caller now passes config.issuer as ground truth, not
  doc.issuer.
- tests: cover three new OIDC callback failure paths (no_id_token,
  id_token_invalid, subject_mismatch) and two idempotency caps (key length
  >128 chars returns 400, body >256 KiB skips caching).

server/src/routes/oidc.ts

@@ -112,7 +112,7 @@ router.get('/callback', async (req: Request, res: Response) => {
       tokenData.id_token,
       doc,
       config.clientId,
-      doc.issuer || config.issuer,
+      config.issuer,
     );
     if (idVerify.ok !== true) {
       const reason = 'error' in idVerify ? idVerify.error : 'unknown';