feat: Add multi-provider AI support

[MujahidAbbas]

Summary

• Add support for 8 AI providers: Anthropic, OpenAI, Gemini, Mistral, Groq, DeepSeek, Ollama, OpenRouter
• Only show providers with configured API keys in the UI
• Allow users to select from curated models or enter custom model IDs
• Add project settings modal to change provider/model mid-project
• Provider/model used for each generation is stored in PlanRun/PlanRunStep records

New Files

File	Purpose
app/Enums/AiProvider.php	Provider enum with Prism integration
config/providers.php	Curated model lists per provider
app/Services/ProviderService.php	Runtime provider detection/resolution
app/Jobs/Concerns/ResolvesAiProvider.php	Shared trait for jobs
app/Livewire/Concerns/ManagesProviderSelection.php	Shared trait for Livewire components
app/Livewire/Projects/SettingsModal.php	Project settings modal

Test Plan

• Configure multiple API keys in .env and verify only those appear in dropdown
• Create project with selected provider/model
• Change provider in settings modal and regenerate PRD
• Verify document versions retain correct provider/model history
• Test custom model ID input
• Test with no providers configured (warning shown)