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

Cargo.toml

@@ -3,6 +3,14 @@ name = "invoice-generator"
 version = "0.1.0"
 edition = "2021"
 
+[lib]
+name = "invoice_generator"
+path = "src/lib.rs"
+
+[[bin]]
+name = "invoice-generator"
+path = "src/main.rs"
+
 [dependencies]
 askama = "0.15.5"
 chrono = { version = "0.4.44", features = ["serde"] }
@@ -13,3 +21,7 @@ tokio = { version = "1", features = ["full"] }
 toml = "0.8"
 anyhow = "1"
 bigdecimal = { version = "0.4", features = ["serde"] }
+
+[dev-dependencies]
+tokio-test = "0.4"
+tempfile = "3"