Refactor: Split MigrationDBHandle into ProjectMigrationDBHandle and SharedMigrationDBHandle

[chendingplano]

Summary

Replace the single ApiTypes.MigrationDBHandle with two separate handles that make the migration track ownership explicit:

• ApiTypes.ProjectMigrationDBHandle — used by goose.RunProjectMigrations for project-level schema changes
• ApiTypes.SharedMigrationDBHandle — used by goose.RunSharedMigrations for shared-library schema changes

Background

The system has three goose migration tracks:

Track	Migrator	Location
Project	goose.ProjectMigrator	<project>/server/migrations/
Shared	goose.SharedMigrator	shared/migrations/
AutoTester	goose.AutoTesterMigrator	<project>/server/migrations/autotester/

Previously, a single MigrationDBHandle was connecting to a separate PG_DB_NAME_MIGRATION database. This was incorrect — project tables and shared tables both live in the same database (ProjectDBHandle). Both migration tracks should therefore run against the project DB, differentiated only by their goose version-tracking table names.

Changes Made

shared/go/api/ApiTypes/ApiTypes.go

• Replaced global var MigrationDBHandle *sql.DB with var ProjectMigrationDBHandle *sql.DB and var SharedMigrationDBHandle *sql.DB
• In DatabaseConfig struct: removed MigrationDBHandle *sql.DB and MigrationDBName string; added ProjectMigrationDBHandle *sql.DB and SharedMigrationDBHandle *sql.DB

shared/go/api/ApiUtils/ApiUtils.go

• Removed the separate migration DB connection (no longer opens a connection to PG_DB_NAME_MIGRATION)
• Both ProjectMigrationDBHandle and SharedMigrationDBHandle are now assigned to ProjectDBHandle at initialisation time
• Removed PG_DB_NAME_MIGRATION env var requirement and related validations
• Updated SetConfig to populate both new handles

shared/go/api/databaseutil/databaseutil.go

• Removed MigrationDBHandle.Close() calls from CloseDatabase (closing ProjectDBHandle is sufficient since the migration handles share the same pointer)

Callers updated

• ChenWeb/server/cmd/deepdoc/main.go
• ChenWeb/server/cmd/autotester/main.go
• tax/server/cmd/autotester/main.go
• shared/go/cmd/autotester/main.go
• ChenWeb/server/cmd/config/config.go

Environment Variable Removed

PG_DB_NAME_MIGRATION is no longer required. Any .env files referencing this variable can be cleaned up.