Clarify docs on spawn database

[saward]

Description

Docs did not clearly specify that spawn_database refers to where the spawn metadata is stored. At the same time, have renamed 'database' to 'target' in a bunch of places, so that it's clearer that this is referring to something other than a mysql concept of database or a postgres concept of it.

Also, spawn_database usage was not yet implemented, so this PR implements it.

Breaking change.

Issue

• Issue Link: docs: spawn_database vague documentation #28

[Copilot]

Pull request overview

This PR addresses issue #28 by clarifying documentation on spawn_database and simultaneously renaming the database/DatabaseConfig concept to target/TargetConfig throughout the codebase. The rename makes it clearer that "targets" refer to spawn connection configurations rather than a SQL concept of "database." This is a breaking change to both the TOML config format and the CLI API.

Changes:

• Rename DatabaseConfig → TargetConfig, databases → targets, --database → --target across all Rust source and tests
• Make spawn_database optional (Option<String>) and implement \c spawn_database psql metacommand to redirect migration tracking to a separate database when configured
• Update all documentation, CLI reference pages, and add a new integration test for the spawn_database split-database behavior

Reviewed changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/engine/mod.rs	Renames DatabaseConfig → TargetConfig, spawn_database becomes Option<String>
src/engine/postgres_psql.rs	Adds spawn_db_connect_command() and new build_record_migration_sql method; renames field types
src/config.rs	Renames database/databases to target/targets, updates db_config() to target_config()
src/cli.rs	Renames --database CLI flag to --target
src/commands/migration/mod.rs	Updates to use target_config() and config.target
src/commands/check.rs	Updates to use config.target and target_config()
src/commands/init.rs	Updates generated config to use targets section
src/template.rs	Updates to use target_config()
tests/migration_build.rs	Updates test helper to use TargetConfig and targets
tests/integration_postgres.rs	Updates to use TargetConfig/targets; adds test_spawn_database_config integration test
mise.toml	Adds [tasks.test] task
docs/src/components/cli-options.ts	Renames databaseOption → targetOption
All docs/src/content/docs/ files	Updates all documentation references from databases/--database to targets/--target
README.md	Updates config example from [databases.prod] to [targets.prod]

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[Copilot]

Pull request overview

Copilot reviewed 30 out of 30 changed files in this pull request and generated 1 comment.

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[saward]

Fixes #29