feat: introduce system prompt feature with validation

[chenyme]

Summary

Add global and model-level system prompt support for chat models. Global prompts are configured in conversation settings, model-level prompts are configured in model management, and the resolved prompt is injected only for chat tasks. Providers/models that do not support native system prompts fall back to merging the prompt into the latest user message.

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 frontend && pnpm lint
• git diff --check

Screenshots, API examples, or logs

No screenshots included. This change adds admin configuration fields and backend prompt resolution behavior.

Configuration, migration, and compatibility notes

Adds a new runtime setting: chat.default_system_prompt.

Adds systemPrompt to admin model create/update/list API contracts and generated Swagger docs.

Adds system_prompt to platform model persistence. Existing models default to an empty prompt, so existing behavior is unchanged unless a global or model-level prompt is configured.

User-level system prompts are not included. Prompt precedence is: global default prompt, then model-level prompt.

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.