feat: unify API response envelopes and trace propagation

[Cai-Tang-www]

Summary

• add unified response envelope and stable string error codes
• add request trace middleware, request logging, and global error handling
• propagate trace_id through HTTP, Kafka run messages, and worker logs
• migrate JSON handlers and auth/rbac middleware to unified responses
• convert /api/chat SSE payloads to envelope-based chunk/done/error events
• add tests for response mapping, trace middleware, SSE behavior, auth envelopes, and Kafka trace propagation
• document response contract, SSE format, and error code table in README

Verification

• go test ./...
• go build ./...

Scope

Issue #2: 统一错误码与响应规范（含 trace_id）

[fennoai]

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