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/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>