Resolved conversations need a separate suggestion layer so repeated FAQ
signals can be grouped without creating untrusted knowledge entries.
This PR adds the persistence foundation only; it introduces no
user-facing behavior by itself.
## Closes
-
[CW-7495](https://linear.app/chatwoot/issue/CW-7495/backend-llm-changes-to-make-conversation-faqs-as-signalssuggestions)
(stacked PR 1/3; the issue is complete after the full stack lands)
## What changed
- Added `captain_faq_suggestions` with question, answer, embedding,
source count, and review status.
- Added `captain_faq_observations` to retain conversation-level signals.
- Added Captain assistant, account, and conversation associations.
- Added vector and lookup indexes for semantic grouping.
## How to test
This layer has no standalone UI behavior. Apply the migration and
confirm Captain assistants can persist open FAQ suggestions with
attached conversation observations.
