feat: implement Phase 2 test structure and modules

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/tests/db/mod.rs

@@ -0,0 +1,8 @@
+// Database tests module
+
+#[cfg(test)]
+pub mod item_tests;
+#[cfg(test)]
+pub mod tag_tests;
+#[cfg(test)]
+pub mod meta_tests;