Rate-limit PEXPIRE on entity event streams

[nicholaspcr]

Summary

Backport of https://github.com/TheThingsIndustries/lorawan-stack/pull/4960

The events Redis store issued a PEXPIRE per entity event stream per published event, dominating write load on the events Redis. Streams are now refreshed at most once per events.redis.store.entity-ttl / 2. Retention is unchanged.

Changes

• New pkg/events/redis/expire_limiter.go: a fixed-size, pointer-free table that allows one PEXPIRE per stream per interval. Anything uncertain allows a PEXPIRE, never skips one.
• publish now gates PExpire on the limiter, built in NewPubSub.
• New events.redis.store.expire-limiter-size: streams tracked, 8 bytes each, defaulting to 4194304 (32 MiB). Negative refreshes the TTL on every event.

Testing

Steps

1. go test -race -run TestExpireLimiter ./pkg/events/redis/
2. go test -run XXX -bench BenchmarkExpireLimiter ./pkg/events/redis/
3. tools/bin/mage dev:dbStart && TEST_REDIS=1 go test ./pkg/events/redis/
4. golangci-lint run ./pkg/events/redis/...

Results

Limiter tests pass under -race; allow is 10.28 ns/op, zero allocations (Apple M4); lint reports no new issues.

Regressions

Affects every component publishing events with events.redis.store.enable set, most visibly the Application Server and Network Server. Covered by the existing suite plus limiter tests for counter wrap, eviction, concurrency, and refresh landing inside the jittered TTL.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.