GHSA-92hr-gmr6-h8cp: Etherpad addressed weak token RNG, login timing, plugin path handling, API request handling

Fix: PR #7906 (ether/etherpad). A set of medium/low hardening fixes:

• Weak RNG for tokens (CWE-330): author/session/readonly IDs were generated with Math.random() (client and server). Now use crypto.getRandomValues.
• Login timing / no failure delay (CWE-208/CWE-307): the OIDC interaction login used a non-constant-time password compare with no failure delay. Now uses crypto.timingSafeEqual plus a uniform failure delay; user lookup is own-property only.
• Plugin dependency path handling (CWE-22): plugin dependency names from package.json were used to build filesystem paths without validation (admin-gated install). Now validated against the npm name grammar.
• API parameter pollution (CWE-235): /api/2 merged all request headers into the API field set. Now forwards only authorization, matching the openapi.ts handler.
• Pad-creation side effect: API.appendChatMessage could create arbitrary pads (missing getPadSafe). Now requires the pad to exist.
• Error info disclosure (CWE-209): the admin file server echoed filesystem error detail; now returns a generic message.