Implement complete collection persistence with database-backed survival across server restarts

• Add full multi-table schema for collections with normalized design (collections, collection_templates, collection_items, collection_item_versions)
• Implement collection detection and processing in enhancement pipeline for .insertr-add elements
• Add template extraction and storage from existing HTML children with multi-variant support
• Enable collection reconstruction from database on server restart with proper DOM rebuilding
• Extend ContentClient interface with collection operations and full database integration
• Update enhance command to use engine.DatabaseClient for collection persistence support

internal/content/assets/insertr.css

@@ -827,4 +827,130 @@ body:not(.insertr-edit-mode) .insertr-editing-hover::after {
         margin-right: 0;
         order: -1;
     }
+}
+
+/* =================================================================
+   COLLECTION MANAGEMENT STYLES (.insertr-add)
+   ================================================================= */
+
+/* Collection container when active */
+.insertr-collection-active {
+    outline: 2px dashed var(--insertr-primary);
+    outline-offset: 4px;
+    border-radius: var(--insertr-border-radius);
+}
+
+/* Add button positioned in top right of container */
+.insertr-add-btn {
+    position: absolute;
+    top: -12px;
+    right: -12px;
+    background: var(--insertr-primary);
+    color: var(--insertr-text-inverse);
+    border: none;
+    padding: var(--insertr-spacing-xs) var(--insertr-spacing-sm);
+    border-radius: var(--insertr-border-radius);
+    font-size: var(--insertr-font-size-sm);
+    font-weight: 600;
+    cursor: pointer;
+    z-index: var(--insertr-z-floating);
+    box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
+    transition: all 0.2s ease;
+}
+
+.insertr-add-btn:hover {
+    background: var(--insertr-primary-hover);
+    transform: translateY(-1px);
+    box-shadow: 0 4px 8px rgba(0, 0, 0, 0.15);
+}
+
+.insertr-add-btn:active {
+    transform: translateY(0);
+}
+
+/* Item controls positioned in top right corner of each item */
+.insertr-item-controls {
+    position: absolute;
+    top: 8px;
+    right: 8px;
+    display: flex;
+    gap: 2px;
+    opacity: 0;
+    transition: opacity 0.2s ease;
+    z-index: var(--insertr-z-floating);
+}
+
+/* Individual control buttons */
+.insertr-control-btn {
+    width: 20px;
+    height: 20px;
+    background: var(--insertr-bg-primary);
+    border: 1px solid var(--insertr-border-color);
+    border-radius: 3px;
+    font-size: 12px;
+    font-weight: bold;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    color: var(--insertr-text-primary);
+    box-shadow: 0 1px 3px rgba(0, 0, 0, 0.1);
+    transition: all 0.15s ease;
+}
+
+.insertr-control-btn:hover {
+    background: var(--insertr-bg-secondary);
+    border-color: var(--insertr-primary);
+    color: var(--insertr-primary);
+    transform: scale(1.1);
+}
+
+/* Remove button specific styling */
+.insertr-control-btn:last-child {
+    color: var(--insertr-danger);
+}
+
+.insertr-control-btn:last-child:hover {
+    background: var(--insertr-danger);
+    color: var(--insertr-text-inverse);
+    border-color: var(--insertr-danger);
+}
+
+/* Collection items hover state */
+.insertr-collection-active > *:hover {
+    background: rgba(0, 123, 255, 0.03);
+    outline: 1px solid rgba(var(--insertr-primary), 0.2);
+    outline-offset: 2px;
+    border-radius: var(--insertr-border-radius);
+}
+
+/* Show item controls on hover */
+.insertr-collection-active > *:hover .insertr-item-controls {
+    opacity: 1;
+}
+
+/* Responsive adjustments for collection management */
+@media (max-width: 768px) {
+    .insertr-add-btn {
+        position: static;
+        display: block;
+        margin: var(--insertr-spacing-sm) auto 0;
+        width: 100%;
+        max-width: 200px;
+    }
+    
+    .insertr-item-controls {
+        position: relative;
+        opacity: 1;
+        top: auto;
+        right: auto;
+        justify-content: center;
+        margin-top: var(--insertr-spacing-xs);
+    }
+    
+    .insertr-control-btn {
+        width: 32px;
+        height: 32px;
+        font-size: 14px;
+    }
 }