fix(api): drop user-derived prefix from /ingest/json tempfile

CodeQL flagged the (whitelist-sanitised) safe_stem as "uncontrolled
data in path expression". The readable-filename feature was debug
ergonomics, not a contract — the URL and source already live inside
the file body. Use NamedTemporaryFile's random default name instead.

Tests untouched (the contract is the file contents, not the path).

Author: gzileni

knowledge-graph-api/api/routes/ingest.py

@@ -146,10 +146,6 @@ async def ingest_json(body: IngestJsonRequest) -> IngestResult:
     Returns:
         An ``IngestResult`` summary.
     """
-    safe_stem = "".join(
-        c if c.isalnum() or c in "-_ " else "_" for c in body.title
-    )[:60].strip() or "article"
-
     serialised = (
         f"{body.title}\n\n"
         f"{body.body}\n\n"
@@ -159,12 +155,16 @@ async def ingest_json(body: IngestJsonRequest) -> IngestResult:
         f"Published: {body.published_at.isoformat()}\n"
     )
 
+    # NamedTemporaryFile defaults: random suffix in the OS temp dir. We
+    # deliberately do NOT derive the path from request fields — even a
+    # whitelist-sanitised body field would still trip CodeQL's
+    # "uncontrolled data in path expression" check, and the readable
+    # filename is debug-only (the URL/source live in the file body).
     tmp_path: str | None = None
     try:
         with tempfile.NamedTemporaryFile(
             delete=False,
             suffix=".txt",
-            prefix=safe_stem + "_",
             mode="w",
             encoding="utf-8",
         ) as tmp: