## Description
Resolved conversations now preserve historical SLA misses without
allowing their displayed duration to keep growing. Applied SLAs record a
stable completion timestamp that is shared through REST and realtime
payloads, and the dashboard freezes FRT, NRT, and RT misses at that
point.
Legacy completed SLAs without a reliable timestamp remain visible as a
static missed state. Terminal SLAs remain frozen when a conversation is
reopened; a reopen before finalization continues the same SLA without
resetting its deadlines.
### Closes
[CW-7597](https://linear.app/chatwoot/issue/CW-7597/freeze-sla-miss-durations-after-conversation-resolution)
## Type of change
- [x] Bug fix (non-breaking change which fixes an issue)
## How to reproduce
1. Apply an SLA with a resolution-time threshold to a conversation.
2. Let the threshold breach, then resolve the conversation.
3. Observe that the recorded miss duration continues increasing every
minute even though the conversation is resolved.
## What changed
- Added nullable `applied_slas.completed_at` and exposed it as
`sla_completed_at` in conversation, report, and websocket payloads.
- Captured completion before broadcasting resolution and preserved it
for terminal applied SLAs.
- Frozen recorded FRT, NRT, and RT durations in classic and
next-generation conversation labels, including a static fallback for
legacy rows.
- Added a dry-run-first, resumable Rails runner for account-scoped or
explicitly global historical repair without enqueuing jobs or touching
`updated_at`.
Account-scoped production rollout starts with:
```sh
ACCOUNT_ID=168154 bundle exec rails runner script/backfill_applied_sla_completed_at.rb
ACCOUNT_ID=168154 APPLY=true bundle exec rails runner script/backfill_applied_sla_completed_at.rb
```
## How Has This Been Tested?
- Verified resolution stamping, nonterminal reopen clearing, and
terminal reopen preservation.
- Verified dry-run, apply, account/global scope, resume, skip,
idempotency, and timestamp-preserving backfill behavior.
- Verified all three miss types freeze and existing conversation-card
behavior remains intact.
- 71 focused RSpec examples and 37 focused Vitest examples pass.
- RuboCop, ESLint, and diff checks pass.
## Checklist:
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules
---------
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
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