Split out db logic

[eupn]

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[Copilot]

Pull request overview

This PR refactors the stats database handling by extracting SQLite logic out of the CashCode driver into a dedicated CashCodeDb module, and wires the shared DB handle into both the bill acceptor and coin acceptor paths.

Changes:

• Added a new CashCodeDb module to own DB connection setup and persistence routines.
• Updated CashCode and the bill acceptor initialization to use an injected CashCodeDb instead of opening the DB internally.
• Added coin persistence (and a payment_log) by recording accepted coin events into the stats DB.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/main.rs	Opens a shared CashCodeDb and passes it into bill/coin acceptor initialization; records coins on acceptance.
src/cashcode.rs	Removes embedded SQLite logic and delegates bill stats operations to CashCodeDb.
src/cashcode_db.rs	Introduces a new DB wrapper with schema init and helpers to record bills/coins and compute totals.

Comments suppressed due to low confidence (1)

src/cashcode_db.rs:79

• record_coin performs multiple writes (updating accepted_coins and inserting into payment_log) without a transaction. Using a transaction here avoids partial updates if one of the statements fails mid-way.

    pub fn record_coin(&self, value: i32) -> Result<(), CashCodeError> {
        let db = self.conn.lock().unwrap();
        db.execute(
            "INSERT INTO accepted_coins (nominal, quantity) VALUES (?1, 1)
             ON CONFLICT(nominal) DO UPDATE SET quantity = quantity + 1",
            [value],
        )?;

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