fix(tests): move memory model tests to avoid import conflicts

Moved tests/unit/models/memory/ to tests/models/memory/ to avoid
Python import path conflicts when pytest collects all tests.

The conflict was caused by tests/models/ and tests/unit/models/ both
having __init__.py files, causing Python to confuse app.models.memory
imports.

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

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

backend/tests/models/memory/__init__.py

@@ -0,0 +1,2 @@
+# tests/unit/models/memory/__init__.py
+"""Unit tests for memory database models."""