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

Pure code movement. Every declaration of
internal/handlers/source_management.go moves unchanged into one of:
webhook_list.go, webhook_create.go, webhook_detail.go, webhook_edit.go
and webhook_delete.go for the webhook pages; event_log.go for the event
log; entrypoint.go for the entrypoint handlers; target_create.go,
target_delete.go and target_toggle.go for the target handlers; and
shared.go for the helpers several of them use. Only each file's package
line and imports are new. The README and a middleware comment that
named the removed file now name the new ones.

Model: opus-5-5

internal/middleware/middleware.go

@@ -131,8 +131,9 @@ const (
 	//
 	//   - Lines carrying an AUTHENTICATED operator's own input, which
 	//     are not truncated at all: the webhook name on "webhook
-	//     created" and the target host on "target URL blocked by SSRF
-	//     protection" (both internal/handlers/source_management.go),
+	//     created" (internal/handlers/webhook_create.go) and the target
+	//     host on "target URL blocked by SSRF protection"
+	//     (internal/handlers/target_create.go),
 	//     and target_name in internal/delivery/engine.go and
 	//     target_http.go. Each is bounded only by the 1 MB form body
 	//     cap, so a 100 KB name writes one line of roughly 600 KB.