test: split oversized database and workflow test modules

[adrunkhuman]

Goal

Split the remaining oversized test modules into smaller contract-focused files so v3 work stays reviewable.

Context

After the v3 season/scoring work, several test files are now large enough to slow navigation and make focused reviews harder:

• tests/test_database.py (~2.1k lines)
• tests/test_admin_panel_fixtures.py (~1.4k lines)
• tests/test_user_commands.py (~1.1k lines)

Test files can be larger than production modules, but these now mix multiple contracts and setup styles.

Scope

Prefer small, mechanical splits with no behavior changes:

• split tests/test_database.py by repository/contract area, for example seasons, fixtures, scoring, schema validation, predictions
• consider splitting tests/test_admin_panel_fixtures.py by fixture creation/edit/delete/season/scoring behaviors
• consider splitting tests/test_user_commands.py by command surface if it remains difficult to navigate
• keep shared fixtures/helpers in conftest.py or focused helper modules only when reuse is real

Non-goals

• do not rewrite test behavior just to split files
• do not add broad abstractions or helper frameworks
• do not weaken coverage or turn behavior tests into mock-call choreography

Acceptance criteria

• tests are split into focused modules with clear names
• full suite still passes
• no production code changes unless needed for imports