Remove an unnecessary "Optional" type hint

This was causing mypy 1.13 to throw an error like this:

src/cyhy_cvesync/main.py:63: error: Argument 3 to "process_urls" has incompatible type "int | None"; expected "int"  [arg-type]

The current version of mypy used in by pre-commit is 1.10.0 and it was NOT flagging this error, but my local mypy 1.13.0 was so I went ahead and fixed it.  All pytests still pass after this change.

src/cyhy_cvesync/models/config_model.py

@@ -31,7 +31,7 @@ class CVESync(BaseModel):
         None,
         description="Logging level",
     )
-    url_concurrency: Optional[int] = Field(
+    url_concurrency: int = Field(
         default=10,
         description="Number of concurrent URL requests to fetch and process CVE data",
     )