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/auth_service.py

@@ -10,8 +10,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.config import settings
 from app.dependencies import JWT_ALGORITHM
 from app.exceptions import AuthenticationError, AuthorizationError, DuplicateError
-from app.models.settings import SystemSetting
 from app.models.user import User
+from app.services import settings_store
 
 
 async def register(db: AsyncSession, username: str, password: str) -> User:
@@ -21,10 +21,7 @@ async def register(db: AsyncSession, username: str, password: str) -> User:
     and creates a user with role='user' and has_access=False.
     """
     # Check registration toggle
-    result = await db.execute(
-        select(SystemSetting).where(SystemSetting.key == "registration_enabled")
-    )
-    setting = result.scalar_one_or_none()
+    setting = await settings_store.get_setting(db, "registration_enabled")
     if setting is not None and setting.value.lower() == "false":
         raise AuthorizationError("Registration is closed")