feat(im): add file_processing_strategy for IM channels — support direct-to-conversation file handling

[Libres-coder]

Summary

IM channels currently only support uploading files to a knowledge base (kb strategy). If no knowledge_base_id is configured, files are rejected with an error. This PR adds a file_processing_strategy field with two modes:

• kb (default) — Upload files to knowledge base (existing behavior)
• direct — Parse file content and inject it directly into the agent conversation context

Changes

Backend (Go):

File	Change
internal/im/types.go	Add FileProcessingStrategy type, constants (FileProcessingStrategyKB, FileProcessingStrategyDirect), and field on IMChannel
internal/im/service.go	Update HandleMessage to dispatch based on strategy; add handleFileMessageDirect and processFileDirect; update buildIMQARequest to accept MessageAttachments
internal/handler/im.go	Accept file_processing_strategy in create/update request structs

Database:

File	Change
migrations/versioned/000040_im_file_strategy.up.sql	Add file_processing_strategy column with CHECK constraint ('kb', 'direct')
migrations/versioned/000040_im_file_strategy.down.sql	Rollback script

Frontend:

File	Change
frontend/src/api/agent/index.ts	Add file_processing_strategy to IMChannel and IMChannelOverview interfaces
frontend/src/components/IMChannelPanel.vue	Add strategy radio group; KB selector shown only when strategy is kb
frontend/src/i18n/locales/zh-CN.ts	Chinese translations
frontend/src/i18n/locales/en-US.ts	English translations
frontend/src/i18n/locales/ru-RU.ts	Russian translations
frontend/src/i18n/locales/ko-KR.ts	Korean translations

How direct strategy works

1. Download file from IM platform
2. For text-based files (txt, md, csv, json, code, etc.) — read directly as UTF-8 text (max 50k chars, truncated with note if exceeded)
3. For binary files (pdf, docx, etc.) — attach metadata note suggesting KB strategy
4. Create MessageAttachment and inject into QA pipeline via buildIMQARequest
5. Supports both streaming and non-streaming output modes

Backward Compatibility

• Default value is 'kb', preserving existing behavior for all existing channels
• BeforeCreate / BeforeSave hooks ensure empty values default to 'kb'
• No breaking changes to API or database schema

[Libres-coder]

ptal,thx!