feat: env-configurable spark-submit binary name

Hardcoding 'spark-submit' as cmd[0] in build_spark_submit_command
breaks for hosts where:
  - both Spark 1.x and 2.x/3.x are installed and 'spark-submit' resolves
    to the wrong one (use 'spark2-submit' or 'spark3-submit' explicitly)
  - the user wants to launch via the PySpark entrypoint ('pyspark')
  - a custom wrapper script sits on PATH (e.g. a credentials-injecting
    'spark-submit-wrapper')

New env var SPARK_EXECUTOR_SPARK_SUBMIT_BIN. Default is 'spark-submit'
(preserves the current behavior for everyone). Override in .env /
docker-compose.yml to change.

common/config.py:
  - new Settings.spark_submit_bin field
  - env-var resolution in from_env() with default 'spark-submit'
  - included in reload() so tests work

spark_executor/core/spark_submit.py:
  - cmd[0] reads settings.spark_submit_bin (was hardcoded 'spark-submit')

.env.example: new section with the override and example values.
docker-compose.yml: forwards the var with the standard 'spark-submit'
default.

Tests: 2 new (settings.spark_submit_bin='spark2-submit', ='pyspark')
plus existing tests updated to use the settings-restore fixture so
mutations don't leak between tests.

165/163 still pass.

spark_executor/core/spark_submit.py

@@ -5,6 +5,7 @@
 """
 import subprocess
 
+from common.config import settings
 from common.logging import logger
 
 
@@ -23,8 +24,12 @@ def build_spark_submit_command(
     num_executors: int,
     spark_conf: dict[str, str] | None = None,
 ) -> list[str]:
+    # cmd[0] is the Spark CLI binary name, configurable via
+    # SPARK_EXECUTOR_SPARK_SUBMIT_BIN. Defaults to 'spark-submit' but
+    # can be 'spark2-submit' (mixed-version hosts), 'pyspark' (PySpark
+    # entrypoint), or a path to a wrapper script.
     cmd = [
-        "spark-submit",
+        settings.spark_submit_bin,
         "--master", master,
         "--deploy-mode", deploy_mode,
         "--queue", queue,