feat(rag): add QA Split-View Chunk-Browser for ingestion verification

New ChunkBrowserQA component replaces inline chunk browser with:
- Document sidebar with live chunk counts per regulation (batched Qdrant count API)
- Sequential chunk navigation with arrow keys (1/N through all chunks of a document)
- Overlap display showing previous/next chunk boundaries (amber-highlighted)
- Split-view with original PDF via iframe (estimated page from chunk index)
- Adjustable chunks-per-page ratio for PDF page estimation

Extracts REGULATIONS_IN_RAG and REGULATION_INFO to shared rag-constants.ts.

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

admin-lehrer/app/api/legal-corpus/route.ts

@@ -119,6 +119,32 @@ export async function GET(request: NextRequest) {
           total_in_page: points.length,
         })
       }
+      case 'regulation-counts-batch': {
+        const col = searchParams.get('collection') || 'bp_compliance_gesetze'
+        const codes = (searchParams.get('codes') || '').split(',').filter(Boolean)
+        const results: Record<string, number> = {}
+        for (let i = 0; i < codes.length; i += 10) {
+          const batch = codes.slice(i, i + 10)
+          await Promise.all(batch.map(async (code) => {
+            try {
+              const res = await fetch(`${QDRANT_URL}/collections/${encodeURIComponent(col)}/points/count`, {
+                method: 'POST',
+                headers: { 'Content-Type': 'application/json' },
+                body: JSON.stringify({
+                  filter: { must: [{ key: 'regulation_code', match: { value: code } }] },
+                  exact: true,
+                }),
+                cache: 'no-store',
+              })
+              if (res.ok) {
+                const data = await res.json()
+                results[code] = data.result?.count || 0
+              }
+            } catch { /* skip failed counts */ }
+          }))
+        }
+        return NextResponse.json({ counts: results })
+      }
       case 'collection-count': {
         const col = searchParams.get('collection') || 'bp_compliance_gesetze'
         const countRes = await fetch(`${QDRANT_URL}/collections/${encodeURIComponent(col)}`, {