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

engine/effects/plugins/__init__.py

@@ -0,0 +1,38 @@
+from pathlib import Path
+
+PLUGIN_DIR = Path(__file__).parent
+
+
+def discover_plugins():
+    from engine.effects.registry import get_registry
+    from engine.effects.types import EffectPlugin
+
+    registry = get_registry()
+    imported = {}
+
+    for file_path in PLUGIN_DIR.glob("*.py"):
+        if file_path.name.startswith("_"):
+            continue
+        module_name = file_path.stem
+        if module_name in ("base", "types"):
+            continue
+
+        try:
+            module = __import__(f"engine.effects.plugins.{module_name}", fromlist=[""])
+            for attr_name in dir(module):
+                attr = getattr(module, attr_name)
+                if (
+                    isinstance(attr, type)
+                    and issubclass(attr, EffectPlugin)
+                    and attr is not EffectPlugin
+                    and attr_name.endswith("Effect")
+                ):
+                    plugin = attr()
+                    if not isinstance(plugin, EffectPlugin):
+                        continue
+                    registry.register(plugin)
+                    imported[plugin.name] = plugin
+        except Exception:
+            pass
+
+    return imported