Add spider functionality for relay crawling, marker management, and new SpiderMode config.

- Introduced the `spider` package for relay crawling, including periodic tasks and one-time sync capabilities.
- Added `SetMarker`, `GetMarker`, `HasMarker`, and `DeleteMarker` methods in the database for marker management.
- Updated configuration with `SpiderMode` and `SpiderFrequency` options to enable and customize spider behavior.
- Integrated `spider` initialization into the main application flow.
- Improved tag handling, NIP-70 compliance, and protected tag validation in event processing.
- Removed unnecessary logging and replaced `errorf` with `fmt.Errorf` for better error handling.
- Incremented version to `v0.5.0`.

pkg/encoders/tag/tags.go

@@ -24,7 +24,8 @@ func NewSWithCap(c int) (s *S) {
 
 func (s *S) Len() int {
 	if s == nil {
-		panic("tags cannot be used without initialization")
+		return 0
+		// panic("tags cannot be used without initialization")
 	}
 	return len(*s)
 }