Add a marten createdb management command

[papilip]

Description

This issue proposes the addition of a new built-in management command, marten createdb, that creates the PostgreSQL (or MySQL) database defined in the project settings — before running marten migrate.

Background

Currently, Marten assumes the target database already exists before running marten migrate. This means developers must create the database manually (e.g. createdb myapp_dev or via psql) before they can run migrations for the first time.

This is a friction point in the onboarding flow:

# Current flow (manual step required)
createdb myapp_development   # ← must be done manually
marten migrate
marten serve

Rails solves this with rails db:create. Django assumes the database exists (no equivalent command). Marten would benefit from a similar convenience command.

Proposed Solution

Introduce a marten createdb management command that:

• Reads the database configuration from Marten.settings.databases (the "default" connection)
• Connects to the database server's system database (postgres for PostgreSQL, mysql for MySQL) to issue a CREATE DATABASE statement
• Is idempotent: if the database already exists, it prints a notice and exits cleanly (no error)
• Handles errors gracefully with a descriptive message

Example output

$ marten createdb
Creating database myapp_development ...
  ✓ Database myapp_development created successfully.

$ marten createdb  # run again
Creating database myapp_development ...
  ✓ Database myapp_development already exists.

Desired onboarding flow

git clone https://github.com/example/myapp.git
cd myapp && cp .env.example .env
shards install
marten createdb   # ← new command
marten migrate
marten serve

Implementation Notes

• The command should live in src/marten/cli/manage/command/create_db.cr
• For PostgreSQL: connect to the postgres system database and run CREATE DATABASE "<name>"
• For MySQL: connect without a database and run CREATE DATABASE IF NOT EXISTS \``
• SQLite: the database file is created automatically by the driver — the command could simply print a notice
• The connection to the system database should reuse the configured host, port, user, and password from settings

Relationship to Existing Issues

This complements #148 (marten cleardatabase): createdb creates the database, cleardatabase resets its content, and migrate populates its schema.

References

• Rails: rails db:create — https://guides.rubyonrails.org/command_line.html
• Issue Add a marten cleardatabase management command #148: Add a marten cleardatabase management command
• Issue Add a marten seed management command #147: Add a marten seed management command