refactor: move effects_plugins to engine/effects/plugins

- Move effects_plugins/ to engine/effects/plugins/
- Update imports in engine/app.py
- Update imports in all test files
- Follows capability-based deps architecture

Closes #27

tests/test_tint_effect.py

@@ -1,6 +1,6 @@
 import pytest
 
-from effects_plugins.tint import TintEffect
+from engine.effects.plugins.tint import TintEffect
 from engine.effects.types import EffectConfig