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/sentiment_provider_service.py

@@ -12,12 +12,11 @@ from __future__ import annotations
 
 import logging
 
-from sqlalchemy import select
 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.services import settings_store
 from app.services.admin_service import update_setting
 
 logger = logging.getLogger(__name__)
@@ -53,15 +52,6 @@ KEY_API_KEY = "sentiment_api_key"
 KEY_BASE_URL = "sentiment_base_url"
 
 
-async def _get_settings_map(db: AsyncSession) -> dict[str, str]:
-    result = await db.execute(
-        select(SystemSetting).where(
-            SystemSetting.key.in_([KEY_PROVIDER, KEY_MODEL, KEY_API_KEY, KEY_BASE_URL])
-        )
-    )
-    return {s.key: s.value for s in result.scalars().all()}
-
-
 def _env_key_for(provider: str) -> str:
     if provider == "openai":
         return settings.openai_api_key or ""
@@ -90,7 +80,7 @@ def _base_url_for(provider: str, stored_base_url: str) -> str:
 
 async def _resolve(db: AsyncSession) -> dict:
     """Resolve effective config from DB > env > default."""
-    stored = await _get_settings_map(db)
+    stored = await settings_store.get_map(db, [KEY_PROVIDER, KEY_MODEL, KEY_API_KEY, KEY_BASE_URL])
 
     provider = (stored.get(KEY_PROVIDER) or "").strip().lower()
     if provider not in VALID_PROVIDERS: