feat(core): refactor linters config and improve code quality (#119)

- [x] Reorganize golangci-lint configuration with documented disable reasons
- [x] Simplify errcheck and revive linter rules with targeted exclusions
- [x] Pre-compile regex patterns in input_validation.go for performance
- [x] Fix type assertions in memory_shard.go and resp.go with safety checks
- [x] Replace string comparison with EqualFold for case-insensitive matching
- [x] Fix loop variable captures in jwk.go and logout.go
- [x] Change high goroutine log level from Info to Debug in autocleanup.go
- [x] Replace deprecated "cancelled" spelling with "canceled" throughout
- [x] Add nolint annotations for intentional unused parameters
- [x] Improve comment formatting for deprecated functions
- [x] Fix comment spelling: "marshalling" → "marshaling"
- [x] Refactor provider warnings formatting in internal/providers/warnings.go
- [x] Simplify metrics summary building in internal/recovery/metrics.go
- [x] Pre-allocate slice in error_recovery.go GetDegradedServices
- [x] Refactor context cancellation checks in redis.go

internal/cache/backends/singleflight.go

@@ -87,7 +87,7 @@ func (s *SingleflightCache) GetOrFetch(ctx context.Context, key string, fetcher
 	// If successful, store in cache
 	if call.err == nil && call.val != nil {
 		// Use a background context for cache storage to ensure it completes
-		// even if the original context is cancelled
+		// even if the original context is canceled
 		storeCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 		_ = s.backend.Set(storeCtx, key, call.val, call.ttl)
 		cancel()