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/admin_service.py

@@ -17,6 +17,7 @@ from app.models.settings import SystemSetting
 from app.models.ticker import Ticker
 from app.models.trade_setup import TradeSetup
 from app.models.user import User
+from app.services import settings_store
 
 logger = logging.getLogger(__name__)
 
@@ -126,18 +127,7 @@ async def reset_password(db: AsyncSession, user_id: int, new_password: str) -> U
 
 async def toggle_registration(db: AsyncSession, enabled: bool) -> SystemSetting:
     """Enable or disable user registration via SystemSetting."""
-    result = await db.execute(
-        select(SystemSetting).where(SystemSetting.key == "registration_enabled")
-    )
-    setting = result.scalar_one_or_none()
-    value = str(enabled).lower()
-
-    if setting is None:
-        setting = SystemSetting(key="registration_enabled", value=value)
-        db.add(setting)
-    else:
-        setting.value = value
-
+    setting = await settings_store.upsert_setting(db, "registration_enabled", str(enabled).lower())
     await db.commit()
     await db.refresh(setting)
     return setting
@@ -155,17 +145,7 @@ async def list_settings(db: AsyncSession) -> list[SystemSetting]:
 
 async def update_setting(db: AsyncSession, key: str, value: str) -> SystemSetting:
     """Create or update a system setting."""
-    result = await db.execute(
-        select(SystemSetting).where(SystemSetting.key == key)
-    )
-    setting = result.scalar_one_or_none()
-
-    if setting is None:
-        setting = SystemSetting(key=key, value=value)
-        db.add(setting)
-    else:
-        setting.value = value
-
+    setting = await settings_store.upsert_setting(db, key, value)
     await db.commit()
     await db.refresh(setting)
     return setting
@@ -309,10 +289,7 @@ async def update_recommendation_config(
 
 
 async def get_ticker_universe_default(db: AsyncSession) -> dict[str, str]:
-    result = await db.execute(
-        select(SystemSetting).where(SystemSetting.key == "ticker_universe_default")
-    )
-    setting = result.scalar_one_or_none()
+    setting = await settings_store.get_setting(db, "ticker_universe_default")
     universe = setting.value if setting else DEFAULT_TICKER_UNIVERSE
     if universe not in SUPPORTED_TICKER_UNIVERSES:
         universe = DEFAULT_TICKER_UNIVERSE
@@ -579,11 +556,7 @@ async def list_jobs(db: AsyncSession) -> list[dict]:
     jobs_out = []
     for name in sorted(VALID_JOB_NAMES):
         # Check enabled setting
-        key = f"job_{name}_enabled"
-        result = await db.execute(
-            select(SystemSetting).where(SystemSetting.key == key)
-        )
-        setting = result.scalar_one_or_none()
+        setting = await settings_store.get_setting(db, f"job_{name}_enabled")
         enabled = setting.value == "true" if setting else True  # default enabled
 
         # Get scheduler job info