This adds a `captain_sessions` table to log every Captain run, starting
with Assistant Responses and Copilot Responses. Each session records the
assistant, model, credits consumed, the FAQs/documents/scenario that
contributed to the response, and the full run context — giving customers
visibility into how a response was generated and giving us durable stats
on credit, FAQ, and document usage (which today only exist as ephemeral
trace metadata and an aggregate account counter).
## What changed
- New `Captain::Session` model with a `session_type` enum (`assistant`,
`copilot`). The subject (`Conversation` / `CopilotThread`) and result
(`Message` / `CopilotMessage`) classes are inferred from the session
type, so the table stores plain `subject_id` / `result_id` ids.
`result_id` is nullable so failed runs that still consumed credits can
be logged.
- Composite indexes on `[session_type, subject_id]`, `[session_type,
result_id]`, and `[account_id, session_type, created_at]` for lookup and
usage-stats queries.
- Factory and model specs.
This PR is schema + model only; the writer/instrumentation that records
sessions from the assistant and copilot flows will follow.
---------
Co-authored-by: Sony Mathew <sony@chatwoot.com>
