refactor: unify env-var config in common/config.py

Before: SPARK_EXECUTOR_DATA_DIR, SPARK_EXECUTOR_JOBS_DIR, and
YARN_RESOURCE_MANAGER_URL were each read directly via os.environ.get()
inside the module that used them. Log level was hardcoded DEBUG in
common/logging.py. There was no single file showing what the full set
of env vars the app reads is.

After: common/config.py defines a single Settings dataclass that
reads all env vars at import time and exposes them as fields on a
module-level singleton. App code uses "from common.config import
settings; settings.data_dir" etc. New SPARK_EXECUTOR_LOG_LEVEL env
var controls stderr + info file verbosity (debug file always gets full
DEBUG).

Improvements:
  - One file lists every env var the app reads (was: grep the codebase)
  - Tests can monkeypatch fields on the settings singleton directly
    instead of monkeypatching the env + reloading
  - Adding a new env var means adding one field in config.py, not
    editing 3+ call sites
  - settings.reload() method for tests that prefer env-var style

Out of scope (kept where they are):
  - GUNICORN_* env vars live in gunicorn.conf.py (gunicorn concept)
  - PYTHONUNBUFFERED in Dockerfile (Python runtime flag)
  - SPARK_SUBMIT_OPTS not in config (JVM flag, not Python)

Test changes:
  - test_job_writer.py: settings.jobs_dir instead of monkeypatching
    SPARK_EXECUTOR_JOBS_DIR
  - test_yarn_client.py: settings.yarn_resource_manager_url instead of
    monkeypatching YARN_RESOURCE_MANAGER_URL
  - test_generate_tool.py: same as job_writer
  - Each test file gets an autouse fixture that snapshots+restores
    settings so one test mutation does not leak into the next

116/116 still pass. Live verified: SPARK_EXECUTOR_LOG_LEVEL=INFO
suppresses DEBUG loguru output as expected.

spark_executor/core/yarn_client.py

@@ -18,10 +18,10 @@ var if unset. This matches the pattern the original Connection.yarn_rm_url
 field was designed for, but no longer requires the `yarn` CLI to interpret it.
 """
 import json
-import os
 
 import httpx
 
+from common.config import settings
 from common.logging import logger
 
 
@@ -35,7 +35,7 @@ class YarnConfigError(YarnError):
 
 def _base_url(yarn_rm_url: str | None) -> str:
     """Resolve and validate the RM URL. Raises YarnConfigError if unusable."""
-    url = yarn_rm_url or os.environ.get("YARN_RESOURCE_MANAGER_URL")
+    url = yarn_rm_url or settings.yarn_resource_manager_url
     if not url:
         raise YarnConfigError(
             "YARN ResourceManager URL is not configured. "