refactor: rename Processor to Webhook and Webhook to Entrypoint

The top-level entity that groups entrypoints and targets is now called
Webhook (was Processor). The inbound URL endpoint entity is now called
Entrypoint (was Webhook). This rename affects database models, handler
comments, routes, and README documentation.

closes #12

internal/database/models.go

@@ -5,8 +5,8 @@ func (d *Database) Migrate() error {
 	return d.db.AutoMigrate(
 		&User{},
 		&APIKey{},
-		&Processor{},
 		&Webhook{},
+		&Entrypoint{},
 		&Target{},
 		&Event{},
 		&Delivery{},