Add comprehensive TDD infrastructure with 45 tests

- Add lib crate exposing modules for integration testing
- Add dev-dependencies: tokio-test 0.4, tempfile
- Refactor parse_csv_fields() as pure function for unit testing
- Add field validation (minimum 16 fields required)
- Fix repository last_insert_id using SELECT LAST_INSERT_ID()
- Add 10 lib tests for CSV parsing and date formatting
- Add 10 config tests for environment configuration
- Add 7 import tests for CSV file parsing
- Add 6 models tests for database structs
- Add 12 repository tests for CRUD operations

tests/common/mod.rs

@@ -0,0 +1,10 @@
+//! Common test utilities.
+//! 
+//! AI AGENT NOTE: This module provides shared test infrastructure
+//! including database helpers and sample data fixtures.
+
+pub mod fixtures;
+pub mod test_db;
+
+pub use fixtures::*;
+pub use test_db::*;