feat: add conversation scheduled runs

[Huixin615]

Refs #3525

Why

This PR adds the first phase of conversation-created scheduled runs.

The issue asks DeerFlow to support scheduled tasks from a conversation. Without this, users cannot ask the agent to run a reminder or follow-up task at a future time, and browser users need to manually refresh to see scheduled results.

This implementation focuses on one-time scheduled runs first, keeping recurring schedules out of scope for a follow-up discussion.

What changed

• Added a built-in schedule_task tool for creating one-time scheduled tasks from the current conversation.
• Added persistent scheduled task storage backed by the existing sqlite/postgres persistence layer.
• Added a Gateway-local scheduler service that polls due tasks, claims them with a DB lease, and executes them through the normal DeerFlow run path.
• Added thread-scoped SSE events so the web chat can refresh scheduled run results without requiring a browser refresh.
• Added scheduled task management APIs for listing, reading, and cancelling scheduled tasks.
• Added support for relative delays via delay_seconds, so requests like "remind me in 3 minutes" do not require the model to know the current clock time.
• Hid internal scheduled-run trigger messages from the chat UI to avoid exposing scheduler prompts to users.
• Fixed /workspace/chats/new routing so users can reliably open a new chat route.
• Added scheduler configuration, documentation, and regression tests.

The scheduler is disabled by default:

scheduler:
  enabled: false
It requires database.backend to be sqlite or postgres because scheduled tasks need durable persistence.
## Surface area

Surface area

• Frontend UI — page / component / setting / interaction under frontend/
• Backend API — endpoint / SSE event / request-response shape under backend/app
• Agents / LangGraph — agent node, graph wiring, langgraph.json, or prompt change
• Sandbox — docker/ or sandboxed execution
• Skills — change under skills/
• Dependencies — new/upgraded entry in backend/pyproject.toml or frontend/package.json (say what it buys us)
• Default behavior change — changes existing behavior without the user opting in (default model, default setting, data shape)
• Docs / tests / CI only — no runtime behavior change

Screenshots / Recording

Bug fix verification

N/A - this is primarily a feature PR.

Validation

Ran:

cd backend && uv run ruff check app packages/harness/deerflow tests/test_scheduled_tasks.py
cd backend && uv run ruff format --check app packages/harness/deerflow tests/test_scheduled_tasks.py
cd backend && uv run pytest
cd frontend && pnpm check
git diff --cached --check

Results:
Backend full test suite: 4392 passed, 16 skipped
Backend ruff check: passed
Backend ruff format check: passed
Frontend lint/typecheck: passed
Cached diff whitespace check: passed

[WillemJiang]

It will be addressed in #1092