Multi-engine support (MySQL, SQL Server, etc.)

[cl8dep]

Summary

The tool is fully coupled to PostgreSQL through Npgsql and pg_dump/pg_restore. There is no abstraction layer for the database provider, migration backend, or SQL dialect. Supporting additional engines requires a full refactor.

Current Limitation

• All connections use new NpgsqlConnection(...) directly in Sanitizer.cs and SchemaValidator.cs
• Migration is hardcoded to spawning pg_dump and pg_restore subprocesses
• Schema introspection queries are PostgreSQL-specific (information_schema, Npgsql types)
• Sanitization SQL uses PostgreSQL-specific functions (e.g., encode(sha256(...), 'hex'))

Proposed Approach

Introduce three interfaces:

interface IDbProvider       // CreateConnection(), GetColumnInfoQuery(), etc.
interface IMigrationBackend // DumpAsync(), RestoreAsync()
interface ISqlDialect       // HashExpression(), ConcatExpression(), etc.

Register the correct implementation via DI based on a engine: postgres | mysql | sqlserver config field.

Acceptance Criteria

• PostgreSQL continues to work as before
• At least one additional engine (e.g., MySQL) can be configured
• All engine-specific SQL is isolated behind the dialect interface
• Tests cover the abstraction layer