fix: configurable internal API URL and upload size enforcement

H4: add internal_api_base_url setting to config.py (default http://localhost:8888,
    env-overridable via INTERNAL_API_BASE_URL); replace all 5 hardcoded base_url
    strings in chat_service.py.

H6: add post-read size check in both Excel and STEP upload handlers;
    raises HTTP 413 when content exceeds settings.max_upload_size_mb.

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

backend/app/api/routers/uploads.py

@@ -107,6 +107,9 @@ async def upload_excel(
     tmp_path = upload_dir / tmp_name
 
     content = await file.read()
+    max_bytes = settings.max_upload_size_mb * 1024 * 1024
+    if len(content) > max_bytes:
+        raise HTTPException(413, detail=f"File exceeds maximum upload size of {settings.max_upload_size_mb} MB")
     tmp_path.write_bytes(content)
 
     try:
@@ -408,6 +411,9 @@ async def upload_step(
         raise HTTPException(400, detail="Only .stp / .step files are accepted")
 
     content = await file.read()
+    max_bytes = settings.max_upload_size_mb * 1024 * 1024
+    if len(content) > max_bytes:
+        raise HTTPException(413, detail=f"File exceeds maximum upload size of {settings.max_upload_size_mb} MB")
     file_hash = hashlib.sha256(content).hexdigest()
 
     # Check dedup