chore: upgrade Python version from 3.10 to 3.13 across the project

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/29bd6108-4094-48ca-8976-10df52774551

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

docs/development/agents.md

@@ -64,7 +64,7 @@ Give your AI assistant context about DMARQ:
 
 ```markdown
 DMARQ is a self-hosted DMARC monitoring platform built with:
-- Backend: FastAPI (Python 3.10+)
+- Backend: FastAPI (Python 3.13+)
 - Database: SQLAlchemy ORM (PostgreSQL/SQLite)
 - Templates: Jinja2 with Tailwind CSS
 - Architecture: RESTful API with server-side rendering

docs/development/contributing.md

@@ -20,7 +20,7 @@ There are many ways to contribute to DMARQ:
 
 ### Prerequisites
 
-- Python 3.9+
+- Python 3.13+
 - Node.js 16+ (for frontend assets)
 - Docker and Docker Compose (recommended)
 - Git

docs/development/testing.md

@@ -27,7 +27,7 @@ backend/app/tests/
 
 ### Prerequisites
 
-- Python 3.10+
+- Python 3.13+
 - Dependencies from `backend/requirements.txt`
 
 ### Installation
@@ -143,7 +143,7 @@ isort backend/app
 Tests run automatically on every push and PR via GitHub Actions (`.github/workflows/test.yml`).
 
 The CI workflow:
-1. Installs dependencies (Python 3.10)
+1. Installs dependencies (Python 3.13)
 2. Runs `pytest` with coverage
 3. Runs linting checks (Black, isort, Flake8, Pylint)
 4. Uploads coverage to Codecov