refactor: drop fetch_url body cap and modernize datetime usage

- fetch_url: remove the 1MB body cap and the streaming helper. The
  manual redirect loop with allowlist re-check (the SSRF fix) is kept
  intact; the body is now read in full via resp.text. Drop the now-
  meaningless `truncated` field from FetchUrlResult and the tests that
  asserted on it. Switched from httpx.stream() back to httpx.get()
  for the redirect loop — cleaner without the body cap.

- datetime: replace deprecated datetime.utcnow() with
  datetime.now(timezone.utc) in submit.py (3 sites) and
  core/job_writer.py (1 site). Update the stale comment in
  core/pending_store.py that referenced the old call.

- Clean up an unused `from common.config import settings` import in
  submit.py that ruff flagged.

Co-Authored-By: Claude <noreply@anthropic.com>

spark_executor/tools/submit.py

@@ -6,9 +6,8 @@
 import os
 import secrets
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 
-from common.config import settings
 from common.logging import logger
 from common.sql_guard import validate_pyspark_code
 from spark_executor.core.connection_store import store as conn_store
@@ -124,7 +123,7 @@ def prepare_submit_job(
         num_executors=num_executors,
         spark_conf=dict(conn.spark_conf),
         extra_args=dict(extra_args or {}),
-        created_at=datetime.utcnow(),
+        created_at=datetime.now(timezone.utc),
         status="PENDING",
     )
     pending_store.save(pending)
@@ -248,7 +247,7 @@ def confirm_submit_job(*, pending_id: str) -> SubmitResult:
                     application_id=application_id,
                     script_path=pending.script_path,
                     queue=pending.queue,
-                    submit_time=datetime.utcnow(),
+                    submit_time=datetime.now(timezone.utc),
                     connection=pending.connection,
                     yarn_rm_url=pending.yarn_rm_url,
                 )
@@ -282,7 +281,7 @@ def confirm_submit_job(*, pending_id: str) -> SubmitResult:
             application_id=application_id,
             script_path=pending.script_path,
             queue=pending.queue,
-            submit_time=datetime.utcnow(),
+            submit_time=datetime.now(timezone.utc),
             connection=pending.connection,
             yarn_rm_url=pending.yarn_rm_url,
         )