Migrate Backstage plugin to proxy architecture with lazy loading

[talissoncosta]

Summary

Refactor the Flagsmith Backstage plugin from backend plugin to proxy-based architecture, following the pattern used by LaunchDarkly, Harness, and other community plugins.

Parent Epic: #5641
Spike: #6359

Current Architecture

Frontend → Backend Plugin → Flagsmith API
           (src/service/)
           (35 API calls for 17 flags)

Target Architecture

Frontend → Backstage Proxy → Flagsmith API
           (/proxy/flagsmith)
           (1 call + lazy load on expand)

Benefits

Aspect	Before	After
Installation	2 packages (frontend + backend)	1 package (frontend only)
Initial API calls	35 (for 17 flags)	1
Configuration	Custom backend config	Standard proxy config
Pattern	Custom	Matches LaunchDarkly, Harness

Implementation Tasks

• Update FlagsmithClient.ts to use proxy (/proxy/flagsmith)
• Add lazy loading methods (getFeatureVersions, getFeatureStates)
• Update FlagsTab.tsx with lazy loading on accordion expand
• Update FlagsmithOverviewCard.tsx for new client
• Delete src/service/ directory (backend plugin)
• Update src/index.ts to remove backend export
• Update package.json to remove backend dependencies
• Update README.md with proxy configuration

Proxy Configuration

Users will configure in app-config.yaml:

proxy:
  endpoints:
    '/flagsmith':
      target: 'https://api.flagsmith.com/api/v1'
      headers:
        Authorization: Api-Key ${FLAGSMITH_API_TOKEN}

Acceptance Criteria

• Plugin works with proxy configuration only
• No backend plugin required
• Lazy loading reduces initial API calls
• README updated with new installation steps