Add OCR Kombi Pipeline: modular 11-step architecture with multi-page support

Phase 1 of the clean architecture refactor: Replaces the 751-line ocr-overlay
monolith with a modular pipeline. Each step gets its own component file.

Frontend: /ai/ocr-kombi route with 11 steps (Upload, Orientation, PageSplit,
Deskew, Dewarp, ContentCrop, OCR, Structure, GridBuild, GridReview, GroundTruth).
Session list supports document grouping for multi-page uploads.

Backend: New ocr_kombi/ module with multi-page PDF upload (splits PDF into N
sessions with shared document_group_id). DB migration adds document_group_id
and page_number columns.

Old /ai/ocr-overlay remains fully functional for A/B testing.

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

admin-lehrer/components/ocr-kombi/KombiStepper.tsx

@@ -0,0 +1,59 @@
+'use client'
+
+import type { PipelineStep } from '@/app/(admin)/ai/ocr-pipeline/types'
+
+interface KombiStepperProps {
+  steps: PipelineStep[]
+  currentStep: number
+  onStepClick: (index: number) => void
+}
+
+export function KombiStepper({ steps, currentStep, onStepClick }: KombiStepperProps) {
+  return (
+    <div className="flex items-center gap-0.5 px-3 py-2.5 bg-white dark:bg-gray-800 rounded-lg border border-gray-200 dark:border-gray-700 overflow-x-auto">
+      {steps.map((step, index) => {
+        const isActive = index === currentStep
+        const isCompleted = step.status === 'completed'
+        const isFailed = step.status === 'failed'
+        const isSkipped = step.status === 'skipped'
+        const isClickable = (index <= currentStep || isCompleted) && !isSkipped
+
+        return (
+          <div key={step.id} className="flex items-center flex-shrink-0">
+            {index > 0 && (
+              <div
+                className={`h-0.5 w-4 mx-0.5 ${
+                  isSkipped
+                    ? 'bg-gray-200 dark:bg-gray-700 border-t border-dashed border-gray-400'
+                    : index <= currentStep ? 'bg-teal-400' : 'bg-gray-300 dark:bg-gray-600'
+                }`}
+              />
+            )}
+            <button
+              onClick={() => isClickable && onStepClick(index)}
+              disabled={!isClickable}
+              className={`flex items-center gap-1 px-2 py-1 rounded-full text-xs font-medium transition-all whitespace-nowrap ${
+                isSkipped
+                  ? 'bg-gray-100 text-gray-400 dark:bg-gray-800 dark:text-gray-600 line-through'
+                  : isActive
+                    ? 'bg-teal-100 text-teal-700 dark:bg-teal-900/40 dark:text-teal-300 ring-2 ring-teal-400'
+                    : isCompleted
+                      ? 'bg-green-100 text-green-700 dark:bg-green-900/40 dark:text-green-300'
+                      : isFailed
+                        ? 'bg-red-100 text-red-700 dark:bg-red-900/40 dark:text-red-300'
+                        : 'text-gray-400 dark:text-gray-500'
+              } ${isClickable ? 'cursor-pointer hover:opacity-80' : 'cursor-default'}`}
+              title={step.name}
+            >
+              <span className="text-sm">
+                {isSkipped ? '-' : isCompleted ? '\u2713' : isFailed ? '\u2717' : step.icon}
+              </span>
+              <span className="hidden lg:inline">{step.name}</span>
+              <span className="lg:hidden">{index + 1}</span>
+            </button>
+          </div>
+        )
+      })}
+    </div>
+  )
+}