Feat: Cluster Management System - Create Clusters & Assign Images

### Describe the feature

## 📋 Description
Currently, PictoPy's AI Tagging can detect and cluster faces automatically, but users **cannot create custom clusters manually** or **assign images to existing clusters**. This limits users' ability to organize photos when AI detection is imperfect or when they want custom organizational structures.

## 🎯 Problem Statement
- No way to create new person/face clusters manually
- Cannot add/remove images to/from existing clusters after AI tagging
- Cluster names are auto-generated IDs (e.g., "Person 6874") with no easy way to customize
- No bulk image assignment UX for cluster management
- Users cannot fix AI mistakes by manually curating clusters

## 💡 Proposed Solution

1. Create Cluster

2. Enhanced Cluster Detail View
Improve the cluster detail page with:
- "Add Images" button opening multi-select gallery picker
- Drag-and-drop image assignment
- Bulk selection with checkbox grid
- Preview of cluster members with remove option
- Image count and search/filter capabilities

3. Backend API Extensions
New/updated endpoints:

### Add ScreenShots

https://github.com/user-attachments/assets/39d30fb2-77fb-46fd-9e99-39e42a61b5a5

### Record

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- [x] I want to work on this issue