feat(ocr): add guided OCR mode using existing annotation regions

When a document has manually drawn annotation boxes, the user can now
enable "Nur annotierte Bereiche" in the OCR trigger panel. The engine
skips layout detection entirely and runs recognition only within the
pre-drawn bounding boxes, preserving manual transcription blocks.

- Python: adds OcrRegion model, extend OcrRequest/OcrBlock; guided
  branch in /ocr/stream groups by page and crops each region
- Engines: add extract_region_text() to both Kraken and Surya
- Java: adds OcrBlockResult.annotationId, OcrClient.OcrRegion,
  TriggerOcrDTO.useExistingAnnotations; OcrAsyncRunner dispatches to
  upsertGuidedBlock when annotationId is present; OcrService threads
  the flag through to runSingleDocument
- TranscriptionService: adds upsertGuidedBlock (creates, updates OCR,
  or preserves MANUAL blocks)
- Frontend: guided OCR toggle in OcrTrigger shown when blocks exist;
  skips destructive-replace confirmation in guided mode

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

frontend/src/lib/components/OcrTrigger.svelte

@@ -7,10 +7,11 @@ import ScriptTypeSelect from './ScriptTypeSelect.svelte';
 interface Props {
 	existingBlockCount: number;
 	storedScriptType: string;
-	onTrigger: (scriptType: string) => void;
+	annotationCount?: number;
+	onTrigger: (scriptType: string, useExistingAnnotations: boolean) => void;
 }
 
-let { existingBlockCount, storedScriptType, onTrigger }: Props = $props();
+let { existingBlockCount, storedScriptType, annotationCount = 0, onTrigger }: Props = $props();
 
 const { confirm } = getConfirmService();
 
@@ -18,10 +19,12 @@ let selectedScriptType: string = $state(
 	untrack(() => (storedScriptType && storedScriptType !== 'UNKNOWN' ? storedScriptType : ''))
 );
 
+let useExistingAnnotations: boolean = $state(false);
+
 async function handleClick() {
 	if (!selectedScriptType) return;
 
-	if (existingBlockCount > 0) {
+	if (!useExistingAnnotations && existingBlockCount > 0) {
 		const confirmed = await confirm({
 			title: m.ocr_confirm_title(),
 			body: m.ocr_confirm_body({ count: String(existingBlockCount) }),
@@ -31,12 +34,27 @@ async function handleClick() {
 		if (!confirmed) return;
 	}
 
-	onTrigger(selectedScriptType);
+	onTrigger(selectedScriptType, useExistingAnnotations);
 }
 </script>
 
 <div class="flex flex-col gap-3">
 	<ScriptTypeSelect bind:value={selectedScriptType} />
+	{#if annotationCount > 0}
+		<div class="flex flex-col gap-1">
+			<label class="flex cursor-pointer items-center gap-2">
+				<input
+					type="checkbox"
+					bind:checked={useExistingAnnotations}
+					class="h-4 w-4 cursor-pointer rounded-sm border-brand-navy/30 accent-brand-navy"
+				/>
+				<span class="font-sans text-sm font-medium text-brand-navy">
+					{m.ocr_use_existing_annotations()}
+				</span>
+			</label>
+			<p class="pl-6 text-xs text-ink-3">{m.ocr_use_existing_annotations_hint()}</p>
+		</div>
+	{/if}
 	<button
 		type="button"
 		disabled={!selectedScriptType}

frontend/src/lib/components/TranscriptionEditView.svelte

@@ -19,7 +19,7 @@ type Props = {
 	onSaveBlock: (blockId: string, text: string) => Promise<void>;
 	onDeleteBlock: (blockId: string) => Promise<void>;
 	onReviewToggle: (blockId: string) => Promise<void>;
-	onTriggerOcr?: (scriptType: string) => void;
+	onTriggerOcr?: (scriptType: string, useExistingAnnotations: boolean) => void;
 	canWrite?: boolean;
 	trainingLabels?: string[];
 	onToggleTrainingLabel?: (label: string, enrolled: boolean) => Promise<void>;
@@ -370,6 +370,7 @@ $effect(() => {
 						<div class="mt-3 max-w-xs">
 							<OcrTrigger
 								existingBlockCount={blocks.length}
+								annotationCount={blocks.length}
 								storedScriptType={storedScriptType}
 								onTrigger={onTriggerOcr}
 							/>