Squash migrations after ULID adoption is complete

[ManukMinasyan]

Overview

Once the ULID migration has been applied to all environments, consolidate incremental migrations into the original create_* files and delete redundant ones. This keeps the migration directory clean and makes migrate:fresh reflect the true schema without running unnecessary alter/drop steps.

Tasks

• Update create_users_table.php to use $table->ulid('id')->primary() and remove sessions FK
• Update create_teams_table.php to include slug column
• Update create_imports_table.php to reflect final schema (with entity_type, status, headers, column_mappings, etc.)
• Update all other create_* migrations to use ULID primary keys and ULID foreign keys
• Delete 2025_12_20_000000_migrate_to_ulid.php
• Delete 2026_02_11_232804_add_slug_to_teams_table.php
• Delete 2026_02_12_222910_expand_imports_table.php
• Delete 2026_02_13_002721_cleanup_imports_table.php
• Delete 2026_02_15_214730_drop_sessions_user_id_foreign_key.php
• Run php artisan migrate:fresh --seed to verify squashed migrations
• Run full test suite to confirm nothing breaks

Success Criteria

• Migration directory contains only create_* and essential update_* migrations
• migrate:fresh produces identical schema to the current incremental chain
• All tests pass
• No environment depends on the deleted incremental migrations