feat: Persistente Sessions (PostgreSQL) + Spaltenerkennung (Step 3)

Sessions werden jetzt in PostgreSQL gespeichert statt in-memory.
Neue Session-Liste mit Name, Datum, Schritt. Sessions ueberleben
Browser-Refresh und Container-Neustart. Step 3 nutzt analyze_layout()
fuer automatische Spaltenerkennung mit farbigem Overlay.

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

klausur-service/backend/main.py

@@ -43,6 +43,7 @@ except ImportError:
     trocr_router = None
 from vocab_worksheet_api import router as vocab_router, set_db_pool as set_vocab_db_pool, _init_vocab_table, _load_all_sessions, DATABASE_URL as VOCAB_DATABASE_URL
 from ocr_pipeline_api import router as ocr_pipeline_router
+from ocr_pipeline_session_store import init_ocr_pipeline_tables
 try:
     from dsfa_rag_api import router as dsfa_rag_router, set_db_pool as set_dsfa_db_pool
     from dsfa_corpus_ingestion import DSFAQdrantService, DATABASE_URL as DSFA_DATABASE_URL
@@ -76,6 +77,13 @@ async def lifespan(app: FastAPI):
     except Exception as e:
         print(f"Warning: Vocab sessions database initialization failed: {e}")
 
+    # Initialize OCR Pipeline session tables
+    try:
+        await init_ocr_pipeline_tables()
+        print("OCR Pipeline session tables initialized")
+    except Exception as e:
+        print(f"Warning: OCR Pipeline tables initialization failed: {e}")
+
     # Initialize database pool for DSFA RAG
     dsfa_db_pool = None
     if DSFA_DATABASE_URL and set_dsfa_db_pool: