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/conftest.py

@@ -6,6 +6,17 @@ import tempfile
 
 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")
+    run_migrations(db_path, "migrations", verbose=False)
+    return get_db(db_path)
+
 
 @pytest.fixture
 def temp_migrations_dir(tmp_path):