Split source_management.go along its CRUD seams (closes #274)

`internal/handlers/source_management.go` had grown to about 2,370 lines holding the webhook, event log, entrypoint and target handlers, so unrelated units had to wait on each other to touch it. It is split, as pure code movement, into files named for what they hold: `webhook_list.go`, `webhook_create.go`, `webhook_detail.go`, `webhook_edit.go`, `webhook_delete.go`, `event_log.go`, `entrypoint.go`, `target_create.go`, `target_delete.go`, `target_toggle.go` and `shared.go`. No function body, signature, comment or behaviour changed. The README's file tree and log-line caveat, and one middleware comment, name the new files.

Model: opus-5-5

internal/handlers/target_toggle.go

@@ -0,0 +1,35 @@
+package handlers
+
+import (
+	"net/http"
+
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// HandleTargetToggle handles toggling a target's active state.
+func (h *Handlers) HandleTargetToggle() http.HandlerFunc {
+	return h.toggleChildResource(
+		"targetID",
+		func(webhookID, childID string) (bool, error) {
+			var tgt database.Target
+
+			err := h.db.DB().Where(
+				"id = ? AND webhook_id = ?",
+				childID, webhookID,
+			).First(&tgt).Error
+			if err != nil {
+				return false, err
+			}
+
+			// Only the active column: saving the whole row would
+			// write back the name and settings read above over an
+			// edit saved since.
+			active := !tgt.Active
+
+			return active, h.db.DB().Model(&tgt).
+				Update("active", active).Error
+		},
+		"failed to toggle target",
+		targetActivated, targetDeactivated,
+	)
+}