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.

tests/unit/test_generate_tool.py

@@ -2,11 +2,29 @@
 import os
 from pathlib import Path
 
+import pytest
+
+from common import config
 from spark_executor.tools import generate
 
 
+@pytest.fixture(autouse=True)
+def _restore_settings():
+    snapshot = config.Settings(
+        data_dir=config.settings.data_dir,
+        jobs_dir=config.settings.jobs_dir,
+        yarn_resource_manager_url=config.settings.yarn_resource_manager_url,
+        log_level=config.settings.log_level,
+    )
+    yield
+    config.settings.data_dir = snapshot.data_dir
+    config.settings.jobs_dir = snapshot.jobs_dir
+    config.settings.yarn_resource_manager_url = snapshot.yarn_resource_manager_url
+    config.settings.log_level = snapshot.log_level
+
+
 def test_generate_writes_code_and_returns_path(monkeypatch, tmp_path: Path):
-    monkeypatch.chdir(tmp_path)
+    config.settings.jobs_dir = str(tmp_path / "data" / "jobs")
     out = generate.generate_job_file(
         "from pyspark.sql import SparkSession\n"
         "spark = SparkSession.builder.getOrCreate()\n"
@@ -20,8 +38,8 @@ def test_generate_writes_code_and_returns_path(monkeypatch, tmp_path: Path):
         assert "SparkSession.builder.getOrCreate()" in f.read()
 
 
-def test_generate_uses_env_var_when_set(monkeypatch, tmp_path: Path):
-    monkeypatch.setenv("SPARK_EXECUTOR_JOBS_DIR", str(tmp_path / "custom"))
+def test_generate_uses_settings_jobs_dir(tmp_path: Path):
+    config.settings.jobs_dir = str(tmp_path / "custom")
     out = generate.generate_job_file("x = 1\n")
     assert out["script_path"].startswith(str(tmp_path / "custom"))
     assert os.path.isfile(out["script_path"])