Implement Delete MCP Server Functionality

## Overview
Add the ability to delete MCP servers from the system with proper cascade handling and soft delete support.

## Business Value
Complete CRUD operations are essential for server management. Users need to remove servers that are no longer in use.

## Technology Stack
- Backend: Go, PostgreSQL
- Frontend: TypeScript, React, Next.js
- Database: PostgreSQL with soft delete patterns

## Skill Level Required
- Beginner to Intermediate
- Understanding of REST APIs and database relationships
- Basic knowledge of Go and React

## Learning Opportunities
- Learn soft delete patterns in database design
- Understand cascade deletion strategies
- Practice implementing RESTful DELETE endpoints
- Gain experience with React confirmation dialogs
- Learn about audit logging best practices

## Impact
This feature completes the CRUD operations for MCP server management, making the platform fully functional for server lifecycle management. Contributors will work on both backend and frontend, providing full-stack experience.

## Current State
- Create, Read, List, Update operations are implemented
- Delete operation is missing
- Database schema supports soft deletes (deleted_at column exists)

## Subtasks
- [ ] Implement soft delete in repository layer
- [ ] Add DELETE endpoint to API
- [ ] Implement cascade deletion for related records
- [ ] Add confirmation dialog in frontend
- [ ] Add delete permission checks
- [ ] Implement bulk delete functionality
- [ ] Add audit logging for deletions

## Implementation Steps
1. Update backend/internal/mcp/handler.go to add DeleteServer method
2. Update backend/internal/database/repository.go with DeleteMCPServer method
3. Add DELETE endpoint: DELETE /api/v1/mcp/servers/:id
4. Update frontend with delete button and confirmation dialog
5. Add cascade deletion for tools, resources, prompts

## Getting Started
1. Review existing CRUD operations in backend/internal/mcp/handler.go
2. Study the database schema in backend/migrations/
3. Check frontend server management components in frontend/src/components/
4. Read CONTRIBUTING.md for development setup instructions

## Acceptance Criteria
- [ ] DELETE /api/v1/mcp/servers/:id endpoint works
- [ ] Soft delete sets deleted_at timestamp
- [ ] Related tools, resources, prompts are cascade deleted
- [ ] Deleted servers don't appear in list views
- [ ] Confirmation dialog prevents accidental deletion
- [ ] Permission checks prevent unauthorized deletion
- [ ] Deletion is logged in audit log

## Testing Requirements
- Unit test for repository delete method
- Integration test for DELETE endpoint
- Test cascade deletion behavior
- E2E test for delete flow

## Estimated Effort
- Hours: 8-12
- Complexity: Medium