perf: speed up test suite with parallel execution

- Enable pytest-xdist with -n auto for parallel test execution
- Consolidate migrated_db fixture in conftest.py (was duplicated in 4 files)
- Remove local fixture definitions and unused imports from test files

Test execution: ~24s -> ~5s (~5x speedup)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_migration_reference_tables.py

@@ -4,18 +4,6 @@
 import apsw
 import pytest
 
-from animaltrack.db import get_db
-from animaltrack.migrations import run_migrations
-
-
-@pytest.fixture
-def migrated_db(tmp_path):
-    """Create a database with migrations applied."""
-    db_path = str(tmp_path / "test.db")
-    migrations_dir = "migrations"
-    run_migrations(db_path, migrations_dir, verbose=False)
-    return get_db(db_path)
-
 
 class TestMigrationCreatesAllTables:
     """Tests that migration creates all reference tables."""