Add Ground Truth regression test system for OCR pipeline

Extract _build_grid_core() from build_grid() endpoint for reuse.
New ocr_pipeline_regression.py with endpoints to mark sessions as
ground truth, list them, and run regression comparisons after code
changes. Frontend button in StepGroundTruth.tsx to mark/update GT.

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

admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx

@@ -43,6 +43,9 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
   const [drawingRegion, setDrawingRegion] = useState(false)
   const [dragStart, setDragStart] = useState<{ x: number; y: number } | null>(null)
   const [dragEnd, setDragEnd] = useState<{ x: number; y: number } | null>(null)
+  const [isGroundTruth, setIsGroundTruth] = useState(false)
+  const [gtSaving, setGtSaving] = useState(false)
+  const [gtMessage, setGtMessage] = useState('')
 
   const leftPanelRef = useRef<HTMLDivElement>(null)
   const rightPanelRef = useRef<HTMLDivElement>(null)
@@ -86,6 +89,10 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
           : `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/image/original`,
       })
 
+      // Check if session has ground truth reference
+      const gt = data.ground_truth
+      setIsGroundTruth(!!gt?.build_grid_reference)
+
       // Load existing validation data
       const valResp = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/reconstruction/validation`)
       if (valResp.ok) {
@@ -196,6 +203,31 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
     }
   }
 
+  // Mark/update ground truth reference
+  const handleMarkGroundTruth = async () => {
+    if (!sessionId) return
+    setGtSaving(true)
+    setGtMessage('')
+    try {
+      const resp = await fetch(
+        `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/mark-ground-truth`,
+        { method: 'POST' }
+      )
+      if (!resp.ok) {
+        const body = await resp.text().catch(() => '')
+        throw new Error(`Ground Truth fehlgeschlagen (${resp.status}): ${body}`)
+      }
+      const data = await resp.json()
+      setIsGroundTruth(true)
+      setGtMessage(`Ground Truth gespeichert (${data.cells_saved} Zellen)`)
+      setTimeout(() => setGtMessage(''), 5000)
+    } catch (e) {
+      setGtMessage(e instanceof Error ? e.message : String(e))
+    } finally {
+      setGtSaving(false)
+    }
+  }
+
   // Handle manual region drawing on reconstruction
   const handleReconMouseDown = (e: React.MouseEvent<HTMLDivElement>) => {
     if (!drawingRegion) return
@@ -570,8 +602,20 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
         <div className="text-sm text-gray-500 dark:text-gray-400">
           {status === 'saved' && <span className="text-green-600 dark:text-green-400">Validierung gespeichert</span>}
           {status === 'saving' && <span>Speichere...</span>}
+          {gtMessage && (
+            <span className={gtMessage.includes('fehlgeschlagen') ? 'text-red-500' : 'text-amber-600 dark:text-amber-400'}>
+              {gtMessage}
+            </span>
+          )}
         </div>
         <div className="flex items-center gap-3">
+          <button
+            onClick={handleMarkGroundTruth}
+            disabled={gtSaving || status === 'saving'}
+            className="px-4 py-2 text-sm bg-amber-600 text-white rounded hover:bg-amber-700 disabled:opacity-50"
+          >
+            {gtSaving ? 'Speichere...' : isGroundTruth ? 'Ground Truth aktualisieren' : 'Als Ground Truth markieren'}
+          </button>
           <button
             onClick={handleSave}
             disabled={status === 'saving'}