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_seeds.py

@@ -1,10 +1,6 @@
 # ABOUTME: Tests for the reference data seeder.
 # ABOUTME: Validates that seeding populates correct data and is idempotent.
 
-import pytest
-
-from animaltrack.db import get_db
-from animaltrack.migrations import run_migrations
 from animaltrack.models.reference import ProductUnit, UserRole
 from animaltrack.repositories import (
     FeedTypeRepository,
@@ -16,14 +12,6 @@ from animaltrack.repositories import (
 from animaltrack.seeds import run_seeds
 
 
-@pytest.fixture
-def migrated_db(tmp_path):
-    """Create a database with migrations applied."""
-    db_path = str(tmp_path / "test.db")
-    run_migrations(db_path, "migrations", verbose=False)
-    return get_db(db_path)
-
-
 class TestSeedCounts:
     """Tests that seeding creates the correct number of records."""