fix: increase timeout and parallelize receipt processing

- ab_ai_bot: raise requests.post timeout 120s -> 600s so long OCR+LLM
  runs don't silently drop the reply in Discuss
- upload: run parse_upload in ThreadPoolExecutor so tesseract OCR
  doesn't block the FastAPI event loop
- expenses_agent: parse all receipts concurrently with asyncio.gather
  (Ollama semaphore caps parallelism at 2); reduces 13-receipt LLM
  time from ~39s sequential to ~20s parallel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

addons/activeblue_ai/models/ab_ai_bot.py

@@ -102,7 +102,7 @@ class AbAiBot(models.Model):
         if session_id:
             payload['session_id'] = session_id
         try:
-            resp = requests.post(url, json=payload, headers=self._build_headers(), timeout=120)
+            resp = requests.post(url, json=payload, headers=self._build_headers(), timeout=600)
             resp.raise_for_status()
             return resp.json()
         except requests.exceptions.Timeout:
@@ -140,7 +140,7 @@ class AbAiBot(models.Model):
 
         try:
             resp = requests.post(url, data=form_data, files=files or [('files', ('empty', b'', 'text/plain'))],
-                                 headers=headers, timeout=120)
+                                 headers=headers, timeout=600)
             resp.raise_for_status()
             return resp.json()
         except requests.exceptions.Timeout: