feat: implement auto generate title logic for conversations

[chenyme]

Summary

Add a user-level chat setting to control automatic conversation title generation.

When enabled, conversations keep the existing AI-generated title behavior. When disabled, the first user message is used as the conversation title, normalized and truncated to the first 20 characters. The metadata refresh flow was also adjusted so newly generated titles are pulled back into the current chat header and sidebar after the first 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 ./internal/application/conversation ./internal/application/usersettings
• cd frontend && pnpm lint
• git diff --check

Screenshots, API examples, or logs

N/A

Configuration, migration, and compatibility notes

Adds user setting key chat.auto_generate_title, defaulting to true.

No database migration is required because the setting uses the existing user settings table. Existing users keep the current AI-generated title behavior unless they disable the new setting.

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.