feat: add animal cohort creation projection and service

Implements Step 3.3: Animal Cohort Creation

- Add AnimalRegistryProjection for animal_registry and live_animals_by_location
- Add EventAnimalsProjection for event_animals link table
- Add IntervalProjection for location and attribute intervals
- Add AnimalService with create_cohort() for coordinating event + projections
- Add seeded_db fixture to conftest.py
- Update projections/__init__.py with new exports

All operations atomic within single transaction. Includes validation for
location (exists, active) and species (exists, active).

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

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

tests/conftest.py

@@ -26,6 +26,15 @@ def temp_migrations_dir(tmp_path):
     return migrations_path
 
 
+@pytest.fixture
+def seeded_db(migrated_db):
+    """Database with migrations and seed data applied."""
+    from animaltrack.seeds import run_seeds
+
+    run_seeds(migrated_db)
+    return migrated_db
+
+
 @pytest.fixture
 def fresh_db_path(tmp_path):
     """Provide a path for a non-existent database file.