feat: track OpenRouter usage

[wjiayis]

Store usage information in database to track user behavior (if user is not BYOK) as a precursor to billing users, e.g.,

[
  {
    _id: ObjectId('69bf6105f1fe4dba906bf408'),
    user_id: ObjectId('6975da46d6096ac1b07342c2'),
    model_slug: 'openai/gpt-4.1',
    hour_bucket: ISODate('2026-03-22T03:00:00.000Z'),
    project_id: '69746440592738ba2708cc34',
    cost: 0.04201,
    updated_at: ISODate('2026-03-22T03:27:01.229Z')
  },
  {
    _id: ObjectId('69bf61a859daabf3de104287'),
    project_id: '69746440592738ba2708cc34',
    hour_bucket: ISODate('2026-03-22T03:00:00.000Z'),
    model_slug: 'openai/gpt-5-nano',
    user_id: ObjectId('6975da46d6096ac1b07342c2'),
    cost: 0.00067965,
    updated_at: ISODate('2026-03-22T03:27:36.861Z')
  },
  {
    _id: ObjectId('69bf64a23b2bb17df83e0add'),
    user_id: ObjectId('6975da46d6096ac1b07342c2'),
    model_slug: 'openai/gpt-5.2',
    hour_bucket: ISODate('2026-03-22T03:00:00.000Z'),
    project_id: '69746440592738ba2708cc34',
    cost: 0.01436225,
    updated_at: ISODate('2026-03-22T03:40:18.488Z')
  }
]

[wjiayis]

For discussion: do we plan to / how do we track token usage for XtraMCP?