refactor: dedupe scheduler logging/runtime, centralize SystemSetting access, fix rankings N+1

Behavior-preserving cleanup (345 tests pass, ruff clean):

- scheduler: replace 62 inline logger.x(json.dumps({...})) calls with a
  _log_event helper, and collapse 11 identical _job_runtime dicts into an
  _idle_runtime() factory over _JOB_NAMES.
- settings: add app/services/settings_store.py (get_setting/get_value/get_map/
  upsert_setting) and route ~13 hand-rolled SystemSetting queries + two
  identical _settings_map helpers through it.
- scoring.get_rankings: collapse the per-ticker N+1 (3-4 queries + a commit each)
  into 2 bulk reads + a single conditional commit; drop the redundant re-fetch.
  Lazy recompute-on-read is preserved. Adds first tests for get_rankings.

Net ~ -245 lines across the touched modules.

app/services/backtest_service.py

@@ -31,8 +31,8 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.config import settings
-from app.models.settings import SystemSetting
 from app.models.ticker import Ticker
+from app.services import settings_store
 from app.services.admin_service import get_activation_config, update_setting
 from app.services.indicator_service import _extract_ohlcv, compute_atr
 from app.services.outcome_service import (
@@ -741,8 +741,7 @@ async def run_and_store(
 
 async def get_backtest_report(db: AsyncSession) -> dict | None:
     """Return the last cached backtest report, or None if never run."""
-    result = await db.execute(select(SystemSetting).where(SystemSetting.key == KEY_REPORT))
-    setting = result.scalar_one_or_none()
+    setting = await settings_store.get_setting(db, KEY_REPORT)
     if setting is None:
         return None
     try: