security: update python-multipart and starlette to fix high-severity vulnerabilities

- Updated python-multipart from >=0.0.24 to >=0.0.27 (fixes 3 DoS alerts)
- Updated starlette from >=0.27.0 to >=0.49.1 (fixes 1 O(n^2) DoS alert)
- Updated in requirements.txt, pyproject.toml, coordinator-api requirements, aitbc-core

This addresses 4 of the 11 high-severity security vulnerabilities reported by GitHub Dependabot.

packages/py/aitbc-core/pyproject.toml

@@ -15,7 +15,7 @@ dependencies = [
     "redis>=5.0.0",
     "pydantic>=2.5.0",
     "structlog>=23.0.0",
-    "starlette>=0.27.0",
+    "starlette>=0.49.1",
 ]
 
 [build-system]