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/model_webhook.go

@@ -1,14 +1,16 @@
 package database
 
-// Webhook represents a webhook endpoint that feeds into a processor
+// Webhook represents a webhook processing unit that groups entrypoints and targets
 type Webhook struct {
 	BaseModel
 
-	ProcessorID string `gorm:"type:uuid;not null" json:"processor_id"`
-	Path        string `gorm:"uniqueIndex;not null" json:"path"` // URL path for this webhook
-	Description string `json:"description"`
-	Active      bool   `gorm:"default:true" json:"active"`
+	UserID        string `gorm:"type:uuid;not null" json:"user_id"`
+	Name          string `gorm:"not null" json:"name"`
+	Description   string `json:"description"`
+	RetentionDays int    `gorm:"default:30" json:"retention_days"` // Days to retain events
 
 	// Relations
-	Processor Processor `json:"processor,omitempty"`
+	User        User         `json:"user,omitempty"`
+	Entrypoints []Entrypoint `json:"entrypoints,omitempty"`
+	Targets     []Target     `json:"targets,omitempty"`
 }