Deprecate dataSource table with dual-write to workspace.databaseSchema (#19059)

## Summary
- Starts deprecation of the `core.dataSource` table by introducing a
dual-write system: `DataSourceService.createDataSourceMetadata` now
writes to both `core.dataSource` and `core.workspace.databaseSchema`
- Migrates read sites (`WorkspaceDataSourceService.checkSchemaExists`,
`WorkspaceSchemaFactory`, `MiddlewareService`,
`WorkspacesMigrationCommandRunner`) to read from
`workspace.databaseSchema` instead of querying the `dataSource` table
- Removes the unused `databaseUrl` field from `WorkspaceEntity` and
drops the column via migration
- Adds a 1.20 upgrade command to backfill `workspace.databaseSchema`
from `dataSource.schema` for existing workspaces

packages/twenty-server/src/engine/core-modules/workspace/workspace.entity.ts

@@ -234,13 +234,9 @@ export class WorkspaceEntity {
   @Column({ default: 1 })
   metadataVersion: number;
 
-  @Field()
-  @Column({ default: '' })
-  databaseUrl: string;
-
-  @Field()
-  @Column({ default: '' })
-  databaseSchema: string;
+  @Field(() => String, { nullable: true })
+  @Column({ type: 'varchar', nullable: true, default: null })
+  databaseSchema: string | null;
 
   @Field()
   @Column({ unique: true })