fix: Dockerfile Python 3.14 + docs version updates (#4162)

## Summary
- Bump `api/Dockerfile` from `python:3.13-slim` to `python:3.14-slim`
(fixes Cloud Build failure — `requires-python >= 3.14` broke `uv sync`
on 3.13)
- Update CI workflows to `python-version: '3.14'` (ci-tests, ci-lint,
impl-generate, impl-repair, sync-postgres)
- Update docs referencing Python 3.12+ to 3.14+ (copilot-instructions,
project-guide, serena memories)
- Update ESLint 9 → 10 in serena code_style memory

## Test plan
- [x] Cloud Build should succeed with `python:3.14-slim`
- [x] CI workflows already tested with Python 3.14 in previous PR

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

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