fix: bump Dockerfile to Python 3.14 to match requires-python

The pyproject.toml now requires Python >=3.14, but the Dockerfile
was still using python:3.13-slim, causing uv sync to fail during
Cloud Build.

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

Author: MarkusNeusinger

api/Dockerfile

@@ -1,5 +1,5 @@
 # Dockerfile for pyplots FastAPI backend on Cloud Run
-FROM python:3.13-slim
+FROM python:3.14-slim
 
 # Set working directory
 WORKDIR /app