feat: standardize on Python 3.13 and add Docker containerization

[ckrough]

Summary

This PR standardizes the project on Python 3.13 and adds production-ready Docker containerization for local testing.

Python 3.13 Standardization

• Updated requires-python = ">=3.13" in pyproject.toml
• Updated ruff target-version = "py313"
• Updated mypy python_version = "3.13"
• Updated both GitHub Actions CI jobs to Python 3.13
• Regenerated uv.lock with correct Python 3.13 constraint
• Updated all documentation (README, CONTRIBUTING, CLAUDE.md, architecture docs, ADRs)

Docker Containerization

New files:

• Dockerfile - Multi-stage production build with Python 3.13-slim
• docker-compose.yml - Container management with named volumes
• .dockerignore - Build optimization
• entrypoint.sh - Proper signal handling for graceful shutdowns

Key features:

• Production-ready multi-stage build (builder + runtime)
• Non-root user (appuser, uid 1000) for security
• Named volumes for data persistence (SQLite + Chroma + documents)
• Health checks with database verification
• ChromaDB native dependencies included
• 640MB final image size

Development workflow:

• Active development: Host with uv run uvicorn --reload
• Docker: Test production build locally before cloud deployment

Health Check Enhancement

• Enhanced /health endpoint to verify database connectivity
• Returns HTTP 503 on dependency failure
• Logs health check failures with structured logging

Documentation Updates

• README.md: Comprehensive Docker deployment section with volume management
• CONTRIBUTING.md: Docker development workflow
• docs/guides/deployment.md: Full Docker deployment guide with backup/restore
• docs/development-standards.md: Docker testing workflow
• docs/increments.md: Added Increment 14 (Docker) and 15 (Cloud Run)
• All Python version references updated to 3.13

Type

• Feature
• Bug fix
• Refactor
• Documentation
• Tests

Testing

Python 3.13 Verification

• ✅ All type checks pass with mypy (Python 3.13)
• ✅ All linting passes with ruff (py313 target)
• ✅ uv.lock regenerated successfully
• ✅ CI configuration uses Python 3.13

Docker Testing

• ✅ Image builds successfully (640MB)
• ✅ Container starts and passes health checks
• ✅ Database connectivity verified in health endpoint
• ✅ ChromaDB native dependencies working
• ✅ Proper signal handling (SIGTERM)
• ✅ Data persists across container restarts
• ✅ Non-root user verified (appuser)

Checklist

• Tests pass
• Types check
• Docs updated
• Lock file regenerated
• Docker image tested locally
• Health checks verified