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/features/flags.go

@@ -173,7 +173,7 @@ func (m *FeatureManager) LoadFromEnv() {
 	for name, flag := range flags {
 		envVar := "FEATURE_" + name
 		if value := os.Getenv(envVar); value != "" {
-			enabled := strings.ToLower(value) == "true" || value == "1"
+			enabled := strings.EqualFold(value, "true") || value == "1"
 			flag.enabled.Store(enabled)
 		}
 	}