feat: add BT-120 VAT exemption reason (backport #247)

[mergify]

Adds a BT-120 textfield for the VAT exemption reason to the E-Invoice Settings: #183

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This is an automatic backport of pull request #247 done by Mergify.

[greptile-apps]

Confidence Score: 5/5

The change is safe to merge; all new code paths are additive and gated behind an optional settings field that defaults to empty.

The BT-120 write is strictly additive — _set_optional_vat_exemption_reason_text is a no-op when the field is blank, so existing invoices are unaffected. The import side stores the parsed value as read-only display data with no downstream side effects. The _set_seller_id getattr fix only guards a previously unguarded attribute access and cannot regress existing behavior.

No files require special attention.

Sequence Diagram

sequenceDiagram
    participant S as E Invoice Settings
    participant G as EInvoiceGenerator
    participant IT as Item-level TradeTax
    participant DT as Doc-level TradeTax
    participant I as EInvoiceImport
    participant TR as EInvoiceTradeTax row

    Note over G: create_einvoice()
    G->>S: get_single_value("vat_exemption_reason_text")
    S-->>G: vat_exemption_reason_text (cached)

    Note over G: _add_line_item() — rate == 0
    G->>IT: "exemption_reason_code = BT-118 code"
    G->>IT: _set_optional_vat_exemption_reason_text(trade_tax)
    IT-->>IT: "exemption_reason = vat_exemption_reason_text (BT-120)"

    Note over G: _add_empty_tax()
    G->>DT: "exemption_reason_code = BT-118 code"
    G->>DT: _set_optional_vat_exemption_reason_text(trade_tax)
    DT-->>DT: "exemption_reason = vat_exemption_reason_text (BT-120)"

    Note over I: parse_tax() — import flow
    I->>TR: "vat_exemption_reason_text = tax.exemption_reason._text (read-only display)"

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[barredterra]

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