feat: add systemPrompt field to conversation and project models

[chenyme]

Summary

Add project mode for conversations. Projects can now be selected from the sidebar, new chats can be created directly inside a project, and each project can define project-level instructions for chat model requests.

Project instructions are layered after platform/model instructions and before request-level format instructions. They apply to chat requests only; image generation, image editing, and other non-chat capabilities are not affected.

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• cd backend && go test ./...
• cd backend && make swagger
• cd frontend && pnpm lint
• cd frontend && pnpm build
• git diff --check

Screenshots, API examples, or logs

N/A

Configuration, migration, and compatibility notes

• Adds system_prompt to chat_conversation_projects through startup migration.
• Updates conversation project API request/response contracts with systemPrompt.
• Regenerated Swagger docs are included.
• Existing projects default to an empty project prompt.
• Project instructions only affect chat model requests and are not injected into image generation or image editing flows.

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including authentication, authorization, provider routing, file processing, billing, and admin APIs where relevant.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.