refactor: opencode event stream integration and SSE aggregator updates

[chriswritescode-dev]

Summary

• Integrate opencode-event-stream transport adapter
• Refactor SSE aggregator with delta support and status synchronization
• Add OpenCode auth service and event stream client
• Update event context and hooks to use new event stream implementation

Changes

• backend/src/index.ts - Add event stream integration
• backend/src/services/opencode/auth.ts - New OpenCode auth service
• backend/src/services/opencode/client.ts - Update client for event stream
• backend/src/services/sse-aggregator.ts - Refactor with delta support
• backend/src/services/schedules.ts - Update schedule service
• backend/test/services/sse-aggregator.test.ts - New aggregator tests
• frontend/src/contexts/EventContext.tsx - Integrate event stream
• frontend/src/hooks/useSSE.ts - Update SSE hook
• frontend/src/lib/opencode-event-stream/openCodeEventStream.ts - Event stream transport

Type of Change

• Refactor

Checklist

• Code follows project style (no comments, named imports)
• TypeScript types are properly defined
• Tests added/updated (80% coverage target)
• `pnpm lint` passes locally