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_entrypoint.go

@@ -0,0 +1,14 @@
+package database
+
+// Entrypoint represents an inbound URL endpoint that feeds into a webhook
+type Entrypoint struct {
+	BaseModel
+
+	WebhookID   string `gorm:"type:uuid;not null" json:"webhook_id"`
+	Path        string `gorm:"uniqueIndex;not null" json:"path"` // URL path for this entrypoint
+	Description string `json:"description"`
+	Active      bool   `gorm:"default:true" json:"active"`
+
+	// Relations
+	Webhook Webhook `json:"webhook,omitempty"`
+}