Build Flask REST API backend

[itdove]

Description

Create Flask REST API endpoints integrating with ConfigLoader.

Tasks

• Implement GET /api/config - Load configuration
• Implement POST /api/config - Save configuration
• Implement POST /api/config/validate - Validate without saving
• Implement POST /api/jira/test-connection - Test JIRA connectivity
• Add error handling and HTTP status codes
• Implement port auto-detection (8765-8865)
• Add browser auto-launch logic
• Add graceful shutdown (Ctrl+C)

Acceptance Criteria

• GET /api/config returns current config as JSON
• POST /api/config validates and saves using ConfigLoader
• POST /api/config/validate returns validation errors
• Server binds to localhost only (security)
• Auto-detects free port
• Browser opens automatically
• Clean shutdown on Ctrl+C

Estimated Effort: 3-5 days

[itdove]

Child of #305