refactor: decouple monitor evaluation execution target

[shuxueshuxue]

Summary

• treat this as the first execution-target decoupling slice for blocked monitor batch-start globalization
• stop monitor evaluation scheduling from carrying a generic request-derived base_url and rename it to explicit execution_base_url
• add a monitor execution-target resolver helper so the web-local batch-start route no longer directly hardcodes request.base_url semantics
• keep batch-start on web_local_router for now; this PR does not globalize that route

Commit Shape

• this PR intentionally keeps a 5-commit sequence to satisfy the current per-PR density rule

Non-scope

• no move of POST /evaluation/batches/{batch_id}/start into the standalone monitor shell yet
• no move of /threads
• no monitor business-logic rewrite
• no chat/runtime changes

Verification

• uv run pytest -q tests/Unit/monitor/test_monitor_detail_contracts.py -k explicit_execution_target
• uv run pytest -q tests/Unit/monitor/test_monitor_execution_target.py
• uv run pytest -q tests/Unit/monitor/test_monitor_evaluation_scheduler_boundary.py
• uv run ruff check backend/monitor/api/http/execution_target.py backend/monitor/api/http/web_local_router.py backend/monitor/infrastructure/web/gateway.py backend/monitor/application/use_cases/evaluation.py backend/monitor/infrastructure/evaluation/evaluation_scheduler.py backend/monitor/infrastructure/evaluation/background_task_scheduler.py backend/monitor/infrastructure/evaluation/evaluation_execution_service.py tests/Unit/monitor/test_monitor_detail_contracts.py tests/Unit/monitor/test_monitor_execution_target.py tests/Unit/monitor/test_monitor_evaluation_scheduler_boundary.py
• git diff --check