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/migrations/002_ocr_pipeline_sessions.sql

@@ -0,0 +1,28 @@
+-- OCR Pipeline Sessions - Persistent session storage
+-- Applied automatically by ocr_pipeline_session_store.init_ocr_pipeline_tables()
+
+CREATE TABLE IF NOT EXISTS ocr_pipeline_sessions (
+    id UUID PRIMARY KEY,
+    name VARCHAR(255) NOT NULL,
+    filename VARCHAR(255),
+    status VARCHAR(50) DEFAULT 'active',
+    current_step INT DEFAULT 1,
+    original_png BYTEA,
+    deskewed_png BYTEA,
+    binarized_png BYTEA,
+    dewarped_png BYTEA,
+    deskew_result JSONB,
+    dewarp_result JSONB,
+    column_result JSONB,
+    ground_truth JSONB DEFAULT '{}',
+    auto_shear_degrees FLOAT,
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP DEFAULT NOW()
+);
+
+-- Index for listing sessions
+CREATE INDEX IF NOT EXISTS idx_ocr_pipeline_sessions_created
+    ON ocr_pipeline_sessions (created_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_ocr_pipeline_sessions_status
+    ON ocr_pipeline_sessions (status);