[Feature] Unify SSE/event-log envelope across workflows and agents

[jerry609]

Goal

Make streaming and event logging use one envelope schema end-to-end.

Scope

• Define shared event envelope schema
• Map Search/DailyPaper/Analyze events to schema
• Ensure trace_id propagation API -> workflow -> agent -> store/event_log
• Add shared frontend parser utility for streamed events

Acceptance

• Same parser handles Search/Daily/Analyze streams
• Event log fields align with SSE envelope
• trace_id is present and queryable through full chain

[jerry609]

Completed in commit 7e3c7b9 on branch feat/harvest-aligned-base-v2.