Migration fixes and improvements, add example project with all fields

[abondar]

Improved number of things around migrations and fixed some bugs

Should resolve #2074 #2068 for most part

• Fixed datetime field migration
• Added some missing functionality
• Improved output of CLI
• Added example with all fields adding/altering/removing
• Refactored sqlite alter table
• Added sqlmigrate command

AI summary:
This pull request introduces a comprehensive example project demonstrating Tortoise ORM's migration system, along with significant improvements to the migration documentation. The example covers a realistic ERP schema with multiple migration operations and reversible migrations. The documentation is updated to clarify data migration procedures, including usage of RunPython and RunSQL, atomicity controls, and guidance for irreversible migrations.

Comprehensive example project:

• Added examples/comprehensive_migrations_project, featuring a realistic ERP schema and 14 reversible migrations covering all field types and operations (CreateModel, AddField, AlterField, RemoveField, RenameField, RunPython, RunSQL, indexes, constraints).
• Provided configuration file config.py for the example project.
• Added initial and subsequent migration files, each demonstrating different migration operations and schema evolution: creating models, adding fields, altering types, and removing fields. [1] [2] [3] [4] [5] [6] [7] [8]

Migration documentation enhancements:

• Expanded docs/migration.rst to explain data migrations, introduce RunPython and RunSQL, provide guidance on choosing between them, and document atomicity controls for migration operations. [1] [2]
• Added FAQ entry describing how to make migrations irreversible by setting reverse_code=None for RunPython or omitting reverse_sql for RunSQL.
• Updated migration package overview and public entry points to include RunSQL alongside RunPython and CreateModel.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing tech/improve-migrations (af7542d) with develop (2aa8e0d)}

Summary

✅ 16 untouched benchmarks

[coveralls]

Pull Request Test Coverage Report for Build 21785189655

Details

• 268 of 546 (49.08%) changed or added relevant lines in 22 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.8%) to 82.228%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/migrations/schema_generator/state.py	18	19	94.74%
tortoise/migrations/schema_editor/mssql.py	4	6	66.67%
tortoise/migrations/schema_editor/mysql.py	2	4	50.0%
tortoise/migrations/writer.py	9	12	75.0%
tortoise/migrations/schema_generator/state_apps.py	44	49	89.8%
tortoise/migrations/schema_editor/base.py	19	28	67.86%
tortoise/migrations/api/sqlmigrate.py	13	33	39.39%
tortoise/migrations/migration.py	16	38	42.11%
tortoise/migrations/executor.py	26	57	45.61%
tortoise/cli/cli.py	30	76	39.47%

Files with Coverage Reduction	New Missed Lines	%
tortoise/migrations/migration.py	5	64.52%

Totals
Change from base Build 21736468513:	-1.8%
Covered Lines:	10156
Relevant Lines:	11882

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💛 - Coveralls