feat: automatic preview generation on database init

Task 7: Batch-Migration Automation
- Add generateMissingPreviews() method to DatabaseManager
- Automatically runs after schema creation
- Finds all images without preview_path
- Generates previews for existing images on startup
- Graceful error handling (won't break server start)
- Progress logging: 'Found X images without preview, generating...'
- No manual script needed - fully automated

Benefits:
- Works on every backend restart
- Incremental (only missing previews)
- Non-blocking database initialization
- Perfect for deployments and updates

backend/src/database/DatabaseManager.js

@@ -34,6 +34,9 @@ class DatabaseManager {
             // Erstelle Schema
             await this.createSchema();
             
+            // Generate missing previews for existing images
+            await this.generateMissingPreviews();
+            
             console.log('✓ Datenbank erfolgreich initialisiert');
         } catch (error) {
             console.error('Fehler bei Datenbank-Initialisierung:', error);
@@ -206,6 +209,68 @@ class DatabaseManager {
             return false;
         }
     }
+
+    // Generate missing previews for existing images
+    async generateMissingPreviews() {
+        try {
+            console.log('🔍 Checking for images without previews...');
+            
+            // Get all images that don't have a preview_path yet
+            const imagesWithoutPreview = await this.all(`
+                SELECT id, group_id, file_name, file_path 
+                FROM images 
+                WHERE preview_path IS NULL OR preview_path = ''
+            `);
+
+            if (imagesWithoutPreview.length === 0) {
+                console.log('✓ All images have previews');
+                return;
+            }
+
+            console.log(`📸 Found ${imagesWithoutPreview.length} image(s) without preview, generating...`);
+
+            const ImagePreviewService = require('../services/ImagePreviewService');
+            const fsp = require('fs').promises;
+            let successCount = 0;
+            let failCount = 0;
+
+            for (const image of imagesWithoutPreview) {
+                try {
+                    // Check if original file exists
+                    const originalPath = ImagePreviewService.getOriginalPath(image.file_name);
+                    await fsp.access(originalPath);
+
+                    // Generate preview
+                    const previewFileName = ImagePreviewService._getPreviewFileName(image.file_name);
+                    const previewPath = ImagePreviewService.getPreviewPath(previewFileName);
+                    
+                    const result = await ImagePreviewService.generatePreview(originalPath, previewPath);
+                    
+                    if (result.success) {
+                        // Update database with preview_path
+                        await this.run(`
+                            UPDATE images 
+                            SET preview_path = ? 
+                            WHERE id = ?
+                        `, [previewFileName, image.id]);
+                        
+                        successCount++;
+                    } else {
+                        console.warn(`  ⚠️  Preview generation failed for ${image.file_name}: ${result.error}`);
+                        failCount++;
+                    }
+                } catch (error) {
+                    console.warn(`  ⚠️  Could not process ${image.file_name}: ${error.message}`);
+                    failCount++;
+                }
+            }
+
+            console.log(`✓ Preview generation complete: ${successCount} success, ${failCount} failed`);
+        } catch (error) {
+            console.warn('⚠️  Preview generation check failed:', error.message);
+            // Don't throw - this shouldn't prevent DB initialization
+        }
+    }
 }
 
 // Singleton Instance