feat: add bieannial and triannial billing frequency (backport #8)

[mergify]

Addition of biennial and triennial billing (every 2 and 3 years).
This was easily possible for the period_type == "start date" but not for "calendar months" type, so just implemented for the first type.
To improve the UI the correct types are displayed dynamically

Reference: AIT-46

---

This is an automatic backport of pull request #8 done by Mergify.

[greptile-apps]

Confidence Score: 5/5

The change is safe to merge — the core date-calculation logic is well-tested, validation guards are in place, and the error-handling path is correctly wired up.

The new Biennial/Triennial logic uses frequency.value directly, which is simple and consistent across all period types. The MULTI_YEAR_FREQUENCIES sentinel and the start_year anchoring formula are both tested with multiple boundary dates. The validate() hook prevents the one known invalid configuration (CalendarMonths + multi-year + no start_date) from being saved. The only remaining items are minor annotation style inconsistencies and an idiomatic None-check that do not affect runtime behaviour.

No files require special attention beyond the minor style notes on simple_subscription.py.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[create_current_invoice / daily scheduler] --> B[get_from_and_to_date]
    B --> C{period_type?}
    C -->|StartDate| D{billing_time?}
    C -->|CalendarMonths| E{billing_time?}
    D -->|AtBeginning| F[get_date_period]
    D -->|AfterEnd| G[get_date_period prev]
    E -->|AtBeginning| H[get_calendar_period]
    E -->|AfterEnd| I[get_calendar_period prev]
    H --> J{freq in MULTI_YEAR?}
    I --> J
    J -->|Yes Biennial/Triennial| K[block_start_year anchor from start_year]
    J -->|No| L[INVOICE_MONTH_MAP lookup]
    K --> M[from=Jan 1 of block_start_year]
    L --> N[from=month start]
    B -->|ValueError| O{silent?}
    O -->|No| P[frappe.throw]
    O -->|Yes| Q[return silently]
    M --> R{start_date > from_date?}
    N --> R
    R -->|Yes| S[skip / throw]
    R -->|No| T[create_invoice]

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A[create_current_invoice / daily scheduler] --> B[get_from_and_to_date]
    B --> C{period_type?}
    C -->|StartDate| D{billing_time?}
    C -->|CalendarMonths| E{billing_time?}
    D -->|AtBeginning| F[get_date_period]
    D -->|AfterEnd| G[get_date_period prev]
    E -->|AtBeginning| H[get_calendar_period]
    E -->|AfterEnd| I[get_calendar_period prev]
    H --> J{freq in MULTI_YEAR?}
    I --> J
    J -->|Yes Biennial/Triennial| K[block_start_year anchor from start_year]
    J -->|No| L[INVOICE_MONTH_MAP lookup]
    K --> M[from=Jan 1 of block_start_year]
    L --> N[from=month start]
    B -->|ValueError| O{silent?}
    O -->|No| P[frappe.throw]
    O -->|Yes| Q[return silently]
    M --> R{start_date > from_date?}
    N --> R
    R -->|Yes| S[skip / throw]
    R -->|No| T[create_invoice]

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