Fix AI provider exclusivity, provider-aware models, and Gemini streaming

[shettydev]

Summary

• Persist preferences.activeProvider (openrouter/gemini) and enforce a single active provider by clearing the other BYOK key on save.
• Make models/settings provider-aware and route the conversation send + queue processing through the active provider (adds Gemini model support).
• Improve streaming reliability: align Gemini SSE events with OpenRouter, add a Redis-backed stream fanout bridge, and add a best-effort web polling fallback.

Tests

• bun run test -- src/modules/ai/__tests__/ai.controller.spec.ts src/modules/conversations/services/__tests__/queue.gemini-stream.spec.ts src/modules/conversations/services/__tests__/queue.service.spec.ts src/modules/conversations/__tests__/conversation.controller.spec.ts (in packages/mukti-api)
• bun run test -- src/lib/hooks/__tests__/use-conversations.spec.ts (in packages/mukti-web)