Add documentation comments to codebase

Prioritize AI agent-friendly comments explaining:
- Data model rationale (customers, cards, transactions relationships)
- Business rules (anonymized cards, customer_number requirements)
- Config file loading order and environment mapping
- Import pipeline phases and CSV column mapping
- Database operation behaviors (upsert, reset implications)
- SQL query rationale and data filtering rules

src/main.rs

@@ -18,6 +18,10 @@ fn fmt(v: f64) -> String {
     format!("{:.2}", v)
 }
 
+/// Normalizes CSV date format and cleans the data.
+/// 
+/// AI AGENT NOTE: Input CSV may have dates in different formats (MM/DD/YYYY or YYYY-MM-DD).
+/// This function standardizes to YYYY-MM-DD HH:MM:SS format for consistent parsing.
 fn clean_csv_file(
     input_path: &Path,
     output_path: &Path,
@@ -233,6 +237,11 @@ struct CustomerTemplate {
     generated_date: String,
 }
 
+/// Parses the --env flag from CLI arguments.
+/// 
+/// AI AGENT NOTE: The --env flag can appear anywhere in the argument list.
+/// Returns the environment and the index of the "--env" flag (for removal).
+/// Defaults to Prod if not specified.
 fn parse_env_flag(args: &[String]) -> (Env, usize) {
     for (i, arg) in args.iter().enumerate() {
         if arg == "--env" && i + 1 < args.len() {
@@ -248,6 +257,10 @@ fn parse_env_flag(args: &[String]) -> (Env, usize) {
     (Env::default(), 0)
 }
 
+/// Removes --env and its value from argument list.
+/// 
+/// AI AGENT NOTE: This allows the --env flag to appear anywhere in the
+/// command without affecting positional argument parsing.
 fn remove_env_flags(args: &[String]) -> Vec<String> {
     let (_, env_idx) = parse_env_flag(args);
     let mut result = Vec::with_capacity(args.len());