fix(litellm-pgvector): vendor the source instead of a remote git build context

The server's Docker/BuildKit couldn't do a git-context build of a public
github.com repo (fails with "could not read Username ... terminal prompts
disabled" — an auth-shaped error for what should be an anonymous clone).
Rather than debug that, vendor litellm-pgvector's small source tree
directly (vendor/litellm-pgvector/, see VENDORED.md for provenance/update
steps) and build from the local path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

vendor/litellm-pgvector/requirements.txt

@@ -0,0 +1,10 @@
+fastapi==0.104.1
+uvicorn[standard]==0.24.0
+prisma==0.11.0
+python-dotenv==1.0.0
+pydantic>=2.5.0
+psycopg2-binary==2.9.7
+pgvector==0.2.4
+python-multipart==0.0.6
+litellm==1.74.3
+pydantic-settings==2.1.0