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):

tests/test_migration_event_tables.py

@@ -6,18 +6,6 @@ import json
 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 event tables."""

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."""

tests/test_repositories.py

@@ -5,9 +5,7 @@ import time
 
 import pytest
 
-from animaltrack.db import get_db
 from animaltrack.id_gen import generate_id
-from animaltrack.migrations import run_migrations
 from animaltrack.models.reference import (
     FeedType,
     Location,
@@ -26,14 +24,6 @@ from animaltrack.repositories import (
 )
 
 
-@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 now_utc():
     """Current time in milliseconds since epoch."""

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."""