Add configurable identity cache TTL (refresh stale names/avatars)

[Frodotus]

Internal review copy of upstream PR gammons#79 — opened to get a Copilot review.

What

Adds [cache] identity_ttl_days (default 7; 0 = cache forever) so resolved user/bot identities (name + avatar) are re-fetched once stale instead of cached forever.

How

• CacheConfig.IdentityTTLDays; UpsertUser stamps updated_at so freshness is tracked.
• userResolver gains a TTL: Request/RequestBot skip only when cached and fresh; past the TTL they re-fetch (users.info/bots.info).
• Avatar refresh busts the inflight gate, the cached render, and the fetcher's avatar-<id> entry so a changed URL re-downloads.

Testing

go test ./... passes. TestUserResolverFresh (TTL decision incl. 0=off, unstamped=stale) + TestUpsertUserStampsUpdatedAt.

[Copilot]

Pull request overview

Adds a configurable TTL for cached Slack user/bot identities so display names and avatars are periodically re-resolved instead of being cached indefinitely, including invalidation of avatar render and image-cache entries when refreshed.

Changes:

• Introduces [cache] identity_ttl_days config (default 7; 0 disables refresh) and wires it into workspace user resolution.
• Stamps users.updated_at on UpsertUser to track identity freshness, and refreshes identities past the TTL.
• Adds avatar/image-cache invalidation support and new tests covering TTL freshness + updated_at stamping.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
wiki/Configuration.md	Documents new identity_ttl_days cache option.
internal/image/fetcher.go	Adds a Fetcher.Delete helper to evict cached image entries by key.
internal/config/config.go	Adds CacheConfig.IdentityTTLDays with default value.
internal/cache/users.go	Stamps updated_at on UpsertUser when unset to enable TTL freshness checks.
internal/cache/users_ttl_test.go	Adds tests verifying updated_at stamping behavior.
internal/avatar/avatar.go	Adds avatar.Cache.Invalidate to force re-fetch/re-render of avatars on refresh.
cmd/slk/main.go	Adds TTL-aware freshness logic to userResolver and passes configured TTL duration.
cmd/slk/identity_ttl_test.go	Adds unit test for resolver TTL freshness behavior (including ttl=0 and unstamped rows).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Frodotus]

Thanks @copilot — all four addressed in the latest push:

1. fetcher.go — moved Delete below SetAuths so each has its own GoDoc comment immediately above it (the SetAuths doc no longer attaches to Delete).
2. main.go — clamp the identity TTL to 0 for negative config values (if identityTTL < 0 { identityTTL = 0 }), so an accidental negative no longer silently means "never refresh" by coincidence; it's now explicit.
3. users_ttl_test.go — added defer db.Close().
4. users_ttl_test.go — the GetUser("U2") error is now checked with t.Fatalf.

Build + full suite green.