feat(captain): group conversation FAQ signals (2/4) (#14978)

Captain can now turn resolved conversations into FAQ suggestions that
people can review. When a human support agent gives a reusable answer,
Captain saves the question and answer as an observation. Captain groups
matching observations into one suggestion instead of creating a pending
FAQ for every conversation. This PR does not approve or publish FAQs.

PR [#14977](https://github.com/chatwoot/chatwoot/pull/14977) adds the
data model and should be reviewed first. The controller and UI PRs will
add the review flow. The three PRs should merge together.

## Closes


[CW-7495](https://linear.app/chatwoot/issue/CW-7495/backend-llm-changes-to-make-conversation-faqs-as-signalssuggestions).
This is PR 2 of 3. The issue is complete after the full stack lands.

## What changed

1. Runs FAQ generation in the low priority queue after a conversation is
resolved.
2. Reads only customer messages and answers written by human support
agents.
3. Uses the assistant's product details, instructions, response rules,
and guardrails to reject spam and unrelated conversations.
4. Stores each reusable question and answer as an observation.
5. Uses exact text similarity search within the conversation language to
find likely matches.
6. Asks the LLM whether both FAQs ask the same question and give the
same answer.
7. Does not create a new suggestion when an approved FAQ already covers
the observation.
8. Adds matching observations to an existing open suggestion and updates
its source count.
9. Does not suggest the same FAQ again after someone dismisses it.
10. Creates a new open suggestion only when no approved FAQ or existing
suggestion covers the observation.

## How to test

1. Resolve a conversation where a human support agent gives a reusable
answer. Confirm that Captain creates one open suggestion with one
source.
2. Resolve another conversation with the same question and answer.
Confirm that Captain adds a source to the existing suggestion instead of
creating another suggestion.
3. Resolve a conversation that is already covered by an approved FAQ.
Confirm that Captain creates no new suggestion.
4. Dismiss a suggestion, then resolve another conversation with the same
question and answer. Confirm that Captain does not suggest the FAQ
again.
5. Resolve a spam or unrelated conversation. Confirm that Captain
creates no suggestion.

---------

Co-authored-by: Sony Mathew <sony@chatwoot.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

lib/redis/redis_keys.rb

@@ -89,6 +89,7 @@ module Redis::RedisKeys
   WHATSAPP_MESSAGE_MUTEX = 'WHATSAPP_MESSAGE_CREATE_LOCK::%<inbox_id>s::%<sender_id>s'.freeze
   CRM_PROCESS_MUTEX = 'CRM_PROCESS_MUTEX::%<hook_id>s'.freeze
   CAPTAIN_DOCUMENT_SYNC_MUTEX = 'CAPTAIN_DOCUMENT_SYNC_LOCK::%<document_id>s'.freeze
+  CAPTAIN_CONVERSATION_FAQ_MUTEX = 'CAPTAIN_CONVERSATION_FAQ_LOCK::%<assistant_id>s::%<language>s'.freeze
 
   ## Auto Assignment Keys
   # Track conversation assignments to agents for rate limiting