Add v2 discovery API

A Frappe site can now describe its own RPC surface — the primitive that self-configuring clients and AI agents need, even if the contract still says "expect no guarantees."

Until now, finding out which whitelisted methods a Frappe site actually exposes meant grepping app source for @frappe.whitelist or reading each app's docs — there was no way for an API client (or an AI agent) to ask the site itself. This PR adds a native discovery surface to the v2 API that answers exactly that question, machine-readably.

What changed

• New frappe/api/discovery.py wired into four routes: /api/v2/discovery (root document), /discovery/search?q=, /discovery/method (full index), and /discovery/method/{path} (per-method detail with HTTP verbs, params, defaults, type annotations, and docstring).
• The index is built by AST-scanning every installed app for module-level @frappe.whitelist functions in a background job, then cached in Redis for an hour — a cold cache returns 503 with "try again in a few seconds" instead of blocking the request.
• API-type Server Scripts are listed too, but only for users who can read Server Script; the whole surface is gated to the Developer role via frappe.only_for.
• Explicitly unstable and undocumented (no-docs): internal plumbing for v2, with OpenAPI output deliberately left to apps like frappe_openapi.

GET /api/v2/discovery

{
  "type": "discovery",
  "resources": {"methods": 1450},
  "links": {
    "search": "/api/v2/discovery/search?q={query}",
    "methods": "/api/v2/discovery/method",
    "method": "/api/v2/discovery/method/{method}"
  }
}