feat: add third-party backend account support (upstream, modelMap, models)

[imnuke]

Changes

src/account-manager.js

• Added upstream, modelMap, and models fields to the account object
  (both the constructor map and addAccount)
• Added _accountOwnsModel(account, model): when any account declares a
  models list, only accounts that also list the requested model are eligible
  for that request — accounts without a matching entry are skipped at selection
  time, alongside the existing disabled/quota/Fable checks
• Wired the check into _isAvailable after the existing Fable-exhaustion gate

src/server.js

• Per-account upstream URL: account.upstream || upstream so each account can
  target a different base URL without touching the global config
• rewriteModel(body, modelMap): parses the request body as JSON, rewrites
  the model field using the account's modelMap, and re-serializes; returns
  the original buffer unchanged if the model isn't in the map or the body isn't
  valid JSON
• Fixes Content-Length after rewriting: the model name change shifts the body
  size, so the header is updated to match before the request is forwarded

Behavior

Fallback: a third-party account with a higher priority value (lower =
preferred) sits idle while Claude accounts have headroom. When they're
exhausted it takes over, and modelMap translates e.g. claude-sonnet-4-6 →
deepseek-v4-pro[1m] transparently — no change needed in the claude session.

Explicit routing: a models list lets a session opt in to a specific
backend by naming one of its models directly (--model "deepseek-v4-pro[1m]" on launch or /model deepseek-v4-pro[1m] inside a session). Other sessions are unaffected.

Example config

{
  "name": "deepseek",
  "type": "oauth",
  "accessToken": "sk-...",
  "upstream": "https://api.deepseek.com/anthropic",
  "priority": 100,
  "modelMap": {
    "claude-haiku-4-5-20251001": "deepseek-v4-flash",
    "claude-sonnet-4-6": "deepseek-v4-pro[1m]"
  },
  "models": ["deepseek-v4-pro[1m]", "deepseek-v4-pro", "deepseek-v4-flash"]
}