fix(delete): add cascade deletes and fix SvelteKit named action conflict

- Add V14 migration: ON DELETE CASCADE for document_tags and document_receivers
  so deleting a document removes its join-table rows automatically
- Rename default form action to 'update' in the edit page — SvelteKit forbids
  mixing a default action with named actions (was causing 500 on delete)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V14__add_cascade_delete_to_document_join_tables.sql

@@ -0,0 +1,12 @@
+-- Add ON DELETE CASCADE to document_tags and document_receivers so that
+-- deleting a document automatically removes its tag and receiver associations.
+
+ALTER TABLE public.document_tags
+    DROP CONSTRAINT fkc99c5qjulwx9gru07yrhicgd2,
+    ADD CONSTRAINT fkc99c5qjulwx9gru07yrhicgd2
+        FOREIGN KEY (document_id) REFERENCES public.documents(id) ON DELETE CASCADE;
+
+ALTER TABLE public.document_receivers
+    DROP CONSTRAINT fks7t60twjgfmpeqcuc3g0fvjpm,
+    ADD CONSTRAINT fks7t60twjgfmpeqcuc3g0fvjpm
+        FOREIGN KEY (document_id) REFERENCES public.documents(id) ON DELETE CASCADE;

frontend/src/routes/documents/[id]/edit/+page.server.ts

@@ -41,7 +41,7 @@ export async function load({
 }
 
 export const actions = {
-	default: async ({ request, params, fetch }) => {
+	update: async ({ request, params, fetch }) => {
 		// Raw fetch is used here because FormData multipart bodies are passed through
 		// directly from the browser without transformation.
 		const baseUrl = env.API_INTERNAL_URL || 'http://localhost:8080';

frontend/src/routes/documents/[id]/edit/+page.svelte

@@ -64,7 +64,13 @@ function handleDateInput(e: Event) {
 		<div class="mb-6 rounded border border-red-200 bg-red-50 p-4 text-red-700">{form.error}</div>
 	{/if}
 
-	<form method="POST" enctype="multipart/form-data" use:enhance class="space-y-6 pb-20">
+	<form
+		method="POST"
+		action="?/update"
+		enctype="multipart/form-data"
+		use:enhance
+		class="space-y-6 pb-20"
+	>
 		<!-- ── Section 1: Wer & Wann ── -->
 		<div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
 			<h2 class="mb-5 text-xs font-bold tracking-widest text-ink-3 uppercase">