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/ticker_universe_service.py

@@ -20,8 +20,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.config import settings
 from app.exceptions import ProviderError, ValidationError
-from app.models.settings import SystemSetting
 from app.models.ticker import Ticker
+from app.services import settings_store
 
 logger = logging.getLogger(__name__)
 
@@ -268,8 +268,7 @@ async def _fetch_universe_symbols_from_public(universe: str) -> tuple[list[str],
 
 async def _read_cached_symbols(db: AsyncSession, universe: str) -> list[str]:
     key = f"ticker_universe_cache_{universe}"
-    result = await db.execute(select(SystemSetting).where(SystemSetting.key == key))
-    setting = result.scalar_one_or_none()
+    setting = await settings_store.get_setting(db, key)
     if setting is None:
         return []
 
@@ -304,15 +303,7 @@ async def _write_cached_symbols(
         "updated_at": datetime.now(timezone.utc).isoformat(),
     }
 
-    result = await db.execute(select(SystemSetting).where(SystemSetting.key == key))
-    setting = result.scalar_one_or_none()
-    value = json.dumps(payload)
-
-    if setting is None:
-        db.add(SystemSetting(key=key, value=value))
-    else:
-        setting.value = value
-
+    await settings_store.upsert_setting(db, key, json.dumps(payload))
     await db.commit()