feat: modernize Docker Compose setup and simplify installation docs

[tdruez]

Changes

Compose files

• Add compose.yml full stack using pre-built image, picked up by default
• Add compose.dev.yml standalone dev stack with live source mounts and runserver
• Add compose.build.yml builds the app image from source instead of using the pre-built image
• Rename docker-compose.demo.yml to compose.demo.yml
• Remove legacy docker-compose.yml

Installation

• Add install.sh rootless one-liner installer (curl | bash)
  • Downloads compose.yml, docker.env, and database seed data
  • Generates .env with a secure secret key
  • Installs a dejacode wrapper command in ~/.local/bin/
  • Starts the stack and waits for the app to be ready
• dejacode command is a thin docker compose wrapper with uninstall as the only custom subcommand
• Default install directory is ~/.dejacode/

Docs

• Restructure installation.rst into three clear sections:
  • Run with Docker one-liner install for anyone
  • Enterprise deployment domain config, custom nginx via compose.build.yml, hardware requirements (Ubuntu 24.04)
  • Local development fully Docker-based, no local Python or PostgreSQL required
• Remove the "build from source" section enterprise deployments use the pre-built image
• Add "Managing your installation" with common dejacode / docker compose commands