fix: retry words request on 400/404 + add backend diagnostic logging

Frontend: retry /words POST once after 2s delay if it gets 400/404,
which happens when navigating via wizard after container restart
(session cache not yet warm).

Backend: log when session needs DB reload and when dewarped_bgr is missing.

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

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

@@ -105,12 +105,24 @@ export function StepWordRecognition({ sessionId, onNext, goToStep }: StepWordRec
     setGridResult(null)
 
     try {
-      const res = await fetch(
-        `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/words?stream=true&engine=${eng}&pronunciation=${pronunciation}`,
-        { method: 'POST' },
-      )
-      if (!res.ok) {
-        const err = await res.json().catch(() => ({ detail: res.statusText }))
+      // Retry once if initial request fails (e.g. after container restart,
+      // session cache may not be warm yet when navigating via wizard)
+      let res: Response | null = null
+      for (let attempt = 0; attempt < 2; attempt++) {
+        res = await fetch(
+          `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/words?stream=true&engine=${eng}&pronunciation=${pronunciation}`,
+          { method: 'POST' },
+        )
+        if (res.ok) break
+        if (attempt === 0 && (res.status === 400 || res.status === 404)) {
+          // Wait briefly for cache to warm up, then retry
+          await new Promise(r => setTimeout(r, 2000))
+          continue
+        }
+        break
+      }
+      if (!res || !res.ok) {
+        const err = await res?.json().catch(() => ({ detail: res?.statusText })) || { detail: 'Worterkennung fehlgeschlagen' }
         throw new Error(err.detail || 'Worterkennung fehlgeschlagen')
       }