feat(backends/python): use tempfile.gettempdir() instead of hardcoded /tmp (#9629)

Closes #9601

Makes the temporary scratch paths in vllm, vllm-omni, tinygrad, and pocket-tts
backends configurable via the standard TMPDIR env var, instead of always writing
to /tmp. This is a one-line change per call site that calls tempfile.gettempdir()
for the directory and keeps the same filename suffix.

Users who run on systems with a small root partition (or want to relocate scratch
files to a larger volume) can now redirect these by setting TMPDIR
(e.g. TMPDIR=/data/tmp), without affecting the existing LOCALAI_GENERATED_CONTENT_PATH
or LOCALAI_UPLOAD_PATH options that already cover other temp paths.

Files touched:
- backend/python/vllm/backend.py        (1 site: video base64 scratch)
- backend/python/tinygrad/backend.py    (1 site: image fallback dst)
- backend/python/pocket-tts/backend.py  (1 site: tts wav fallback dst)
- backend/python/vllm-omni/backend.py   (2 sites: video + audio scratch)

backend/python/vllm/backend.py

@@ -10,6 +10,7 @@ import os
 import json
 import time
 import gc
+import tempfile
 from typing import List
 from PIL import Image
 
@@ -602,7 +603,7 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
         """
         try:
             timestamp = str(int(time.time() * 1000))  # Generate timestamp
-            p = f"/tmp/vl-{timestamp}.data"  # Use timestamp in filename
+            p = os.path.join(tempfile.gettempdir(), f"vl-{timestamp}.data")
             with open(p, "wb") as f:
                 f.write(base64.b64decode(video_path))
             video = VideoAsset(name=p).np_ndarrays