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/core/job_writer.py

@@ -14,7 +14,7 @@ Resolution order for the output directory:
 """
 import os
 import secrets
-from datetime import datetime
+from datetime import datetime, timezone
 
 from common.config import settings
 from common.logging import logger
@@ -63,7 +63,7 @@ def write_job_file(code: str, jobs_dir: str | None = None) -> str:
 
     effective_dir = resolve_jobs_dir(jobs_dir)
     os.makedirs(effective_dir, exist_ok=True)
-    stamp = datetime.utcnow().strftime("%Y%m%d%H%M%S")
+    stamp = datetime.now(timezone.utc).strftime("%Y%m%d%H%M%S")
     name = f"job_{stamp}_{secrets.token_hex(3)}.py"
     path = os.path.join(effective_dir, name)
     abs_path = os.path.abspath(path)