chore(deps): update tiktoken requirement in /backend

Updates the requirements on [tiktoken](https://github.com/openai/tiktoken) to permit the latest version.
- [Release notes](https://github.com/openai/tiktoken/releases)
- [Changelog](https://github.com/openai/tiktoken/blob/main/CHANGELOG.md)
- [Commits](openai/tiktoken@0.8.0...0.12.0)

---
updated-dependencies:
- dependency-name: tiktoken
  dependency-version: 0.12.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -23,7 +23,7 @@ tree-sitter-typescript>=0.23.0
 # AI/ML
 openai>=1.54.0
 pinecone>=5.0.0
-tiktoken>=0.8.0
+tiktoken>=0.12.0
 
 # Utils
 gitpython>=3.1.0