Create `POST /admin/products_purchased` endpoint

[blau2]

User Story

As a system administrator, I want to create a record of when inventory was purchased so that I can accurately track stock levels and manage spoilage.

Acceptance Criteria

• [ ] A new route is created for POST /admin/products_purchased
• [ ] The new endpoint accepts a JSON array of products purchased in the following format:

  [
  {
  'product_id': Int,
  'quantity': Int,
  'date_purchased': DateTime,
  'purchase_price_cents': Int,
  'tax': Float,
  'date_expiry': DateTime,
  'location_purchased': String?
  },
  {
   ...
  }
  ]

• [ ] Endpoint responds with 200 OK when products successfully added
• [ ] Endpoint responds with 400 Bad Request when body contains malformed JSON
• [ ] Endpoint responds with 422 Unprocessable Entity when input validation fails
• [ ] Endpoint responds with 401 Unauthorized when request is missing auth header
• [ ] Endpoint responds with 403 Forbidden when provided admin auth token is unauthorized (@dzlau to provide details)
• [ ] Products are successfully inserted into the products_purchased table
• [ ] Stock levels are incremented accordingly in Product upon successful insertion
• [ ] The new endpoint, payload format, and response codes are documented in API docs

Additional Considerations

• Can we perform validation to ensure that duplicate requests do not get processed?