feat(conversations): filter person typeahead to correspondents of selected person

Closes #29

Backend:
- Add PersonRepository.findCorrespondents / findCorrespondentsWithFilter
  (native SQL, orders by shared document count DESC, limit 10)
- Add PersonService.findCorrespondents(personId, q) delegating to the
  correct repository method based on whether a query string is present
- Expose GET /api/persons/{id}/correspondents?q= in PersonController

Frontend:
- Add optional restrictToCorrespondentsOf prop to PersonTypeahead
- On focus with the prop set, fetch correspondents immediately (no typing
  required) — opens the dropdown showing top correspondents
- On input with the prop set, hit the correspondents endpoint with q= param
- Without the prop, keep existing /api/persons?q= behaviour unchanged
- Wire the prop bidirectionally in /conversations: sender restricts receiver
  and vice versa

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

frontend/src/lib/components/PersonTypeahead.svelte

@@ -8,10 +8,18 @@ interface Props {
 	label: string;
 	value?: string;
 	initialName?: string;
+	restrictToCorrespondentsOf?: string;
 	onchange?: (value: string) => void;
 }
 
-let { name, label, value = $bindable(''), initialName = '', onchange }: Props = $props();
+let {
+	name,
+	label,
+	value = $bindable(''),
+	initialName = '',
+	restrictToCorrespondentsOf,
+	onchange
+}: Props = $props();
 
 let searchTerm = $derived(initialName);
 
@@ -30,13 +38,24 @@ function handleInput() {
 	clearTimeout(debounceTimer);
 
 	debounceTimer = setTimeout(async () => {
-		if (searchTerm.length < 1) {
-			results = [];
-			return;
-		}
 		loading = true;
 		try {
-			const res = await fetch(`/api/persons?q=${encodeURIComponent(searchTerm)}`);
+			let url: string;
+			if (restrictToCorrespondentsOf) {
+				if (searchTerm.length >= 1) {
+					url = `/api/persons/${restrictToCorrespondentsOf}/correspondents?q=${encodeURIComponent(searchTerm)}`;
+				} else {
+					url = `/api/persons/${restrictToCorrespondentsOf}/correspondents`;
+				}
+			} else {
+				if (searchTerm.length < 1) {
+					results = [];
+					loading = false;
+					return;
+				}
+				url = `/api/persons?q=${encodeURIComponent(searchTerm)}`;
+			}
+			const res = await fetch(url);
 			results = res.ok ? await res.json() : [];
 		} catch (e) {
 			console.error('Suche fehlgeschlagen', e);
@@ -47,6 +66,23 @@ function handleInput() {
 	}, 300);
 }
 
+async function handleFocus() {
+	updateDropdownPosition();
+	showDropdown = true;
+	if (restrictToCorrespondentsOf) {
+		loading = true;
+		try {
+			const res = await fetch(`/api/persons/${restrictToCorrespondentsOf}/correspondents`);
+			results = res.ok ? await res.json() : [];
+		} catch (e) {
+			console.error('Suche fehlgeschlagen', e);
+			results = [];
+		} finally {
+			loading = false;
+		}
+	}
+}
+
 function selectPerson(person: Person) {
 	value = person.id!;
 	searchTerm = `${person.firstName} ${person.lastName}`;
@@ -92,7 +128,7 @@ function clickOutside(node: HTMLElement) {
 		autocomplete="off"
 		bind:value={searchTerm}
 		oninput={handleInput}
-		onfocus={() => { updateDropdownPosition(); showDropdown = true; }}
+		onfocus={handleFocus}
 		placeholder={m.comp_typeahead_placeholder()}
 		class="mt-1 block w-full rounded-md border border-gray-300 p-2 shadow-sm focus:border-blue-500 focus:ring-blue-500"
 	/>

frontend/src/lib/components/PersonTypeahead.svelte.spec.ts

@@ -179,6 +179,69 @@ describe('PersonTypeahead – clearing a selection', () => {
 	});
 });
 
+// ─── Correspondent mode ───────────────────────────────────────────────────────
+
+describe('PersonTypeahead – correspondent mode', () => {
+	it('fetches correspondents immediately on focus when restrictToCorrespondentsOf is set', async () => {
+		mockFetchWithPersons();
+		render(PersonTypeahead, {
+			name: 'receiverId',
+			label: 'Empfänger',
+			restrictToCorrespondentsOf: 'person-a-id'
+		});
+
+		await page.getByPlaceholder('Namen tippen...').click();
+		await waitForDebounce();
+
+		const fetchMock = globalThis.fetch as ReturnType<typeof vi.fn>;
+		expect(fetchMock).toHaveBeenCalledWith(
+			expect.stringContaining('/api/persons/person-a-id/correspondents')
+		);
+	});
+
+	it('shows correspondent results immediately on focus without typing', async () => {
+		mockFetchWithPersons();
+		render(PersonTypeahead, {
+			name: 'receiverId',
+			label: 'Empfänger',
+			restrictToCorrespondentsOf: 'person-a-id'
+		});
+
+		await page.getByPlaceholder('Namen tippen...').click();
+		await waitForDebounce();
+
+		await expect.element(page.getByText('Mustermann, Max')).toBeInTheDocument();
+	});
+
+	it('uses correspondents endpoint with q param when typing', async () => {
+		mockFetchWithPersons();
+		render(PersonTypeahead, {
+			name: 'receiverId',
+			label: 'Empfänger',
+			restrictToCorrespondentsOf: 'person-a-id'
+		});
+
+		await page.getByPlaceholder('Namen tippen...').fill('Anna');
+		await waitForDebounce();
+
+		const fetchMock = globalThis.fetch as ReturnType<typeof vi.fn>;
+		expect(fetchMock).toHaveBeenCalledWith(
+			expect.stringContaining('/api/persons/person-a-id/correspondents?q=Anna')
+		);
+	});
+
+	it('uses normal persons endpoint when restrictToCorrespondentsOf is not set', async () => {
+		mockFetchWithPersons();
+		render(PersonTypeahead, { name: 'senderId', label: 'Absender' });
+
+		await page.getByPlaceholder('Namen tippen...').fill('Anna');
+		await waitForDebounce();
+
+		const fetchMock = globalThis.fetch as ReturnType<typeof vi.fn>;
+		expect(fetchMock).toHaveBeenCalledWith(expect.stringContaining('/api/persons?q=Anna'));
+	});
+});
+
 // ─── Click outside ────────────────────────────────────────────────────────────
 
 describe('PersonTypeahead – click outside', () => {

frontend/src/routes/conversations/+page.svelte

@@ -60,6 +60,7 @@ function toggleSort() {
 					label={m.conv_label_person_a()}
 					bind:value={senderId}
 					initialName={data.initialValues.senderName}
+					restrictToCorrespondentsOf={receiverId || undefined}
 					onchange={() => applyFilters()}
 				/>
 			</div>
@@ -73,6 +74,7 @@ function toggleSort() {
 					label={m.conv_label_person_b()}
 					bind:value={receiverId}
 					initialName={data.initialValues.receiverName}
+					restrictToCorrespondentsOf={senderId || undefined}
 					onchange={() => applyFilters()}
 				/>
 			</div>