perf(channels): refresh_cache merges fresh counts instead of re-listing all channels

[AlexanderMakarov]

Summary

channels_list with refresh_cache=true used to force a synchronous re-list of every channel (conversations.list pagination under Tier-2 rate limiting) — ~55s for a ~1,800-channel workspace, and minutes when Slack throttles. Callers like the sde-automation Slack-unread page only need fresh unread counters, which a single client.counts edge call already provides.

RefreshChannelCounts merges HasUnreads/LastRead/MentionCount from client.counts into the in-memory channels snapshot without touching the cache file, so the TTL clock keeps running and the existing stale-while-revalidate machinery still re-lists the full channel set in the background once the cache expires. When counts are unavailable (no edge client), the handler falls back to the old full synchronous refresh.

Testing

• Unit tests for the counts merge (immutability included), the ErrCountsUnavailable fallback signal, and cache-TTL expiry detection (pkg/provider/api_counts_refresh_test.go).
• Verified live against a real workspace: channels_list refresh_cache=true dropped from ~55s to 0.2s; server log confirms the counts path ran with no fallback and no full re-list.

https://claude.ai/code/session_01JA39im4Ha2g7t4qdLg5Adt

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f9d3140b-8dc2-46fd-8371-077d60c0e35d

📥 Commits

Reviewing files that changed from the base of the PR and between 199fe3f and ba9b8fd.

📒 Files selected for processing (4)

• pkg/handler/channels.go
• pkg/provider/api.go
• pkg/provider/api_counts_refresh_test.go
• pkg/server/server.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/channels-refresh-counts-only

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