[Invoices]: Recurring frequency parity — interval-based schedules (every N days/weeks/months/years) #642

v1 supports 28 recurrence intervals; v2's RecurringFrequency enum (Modules/Invoices/Enums/RecurringFrequency.php) supports 5 (daily, weekly, monthly, quarterly, yearly). v1 users with "every 2 weeks", "every 2 months", "every 15 days", or "every 2 years" billing cannot express their schedules in v2, and v1 data cannot be migrated losslessly.

v1 spec: application/modules/invoices/models/Mdl_invoices_recurring.php:23-52 defines the interval codes — days 1,2,3,4,5,6,15,30; weeks 1-4 (7D/14D/21D/28D); months 1-11; years 1-5 — and application/helpers/date_helper.php:194 (increment_date()) applies them as DateInterval('P' . $code) (e.g. 14D, 3M, 1Y). Epic #52 mentions "Support custom frequencies: every X days/weeks/months" as a bullet but no implementable issue exists.

Part of Epic #52.

Specific

Replace the single coarse frequency value with an interval pair on recurring_invoices:

• frequency_value unsignedTinyInt (>= 1)
• frequency_unit string enum: day | week | month | year
  Next-run advancement becomes next_run_at->add(DateInterval("P{value}{D|W|M|Y}")), matching v1 increment_date() exactly. Keep RecurringFrequency as UI presets that map onto pairs (daily→1/day, weekly→1/week, monthly→1/month, quarterly→3/month, yearly→1/year) plus a "custom" option exposing the two fields; migration backfills existing rows via that map.

Measurable

• Every v1 code from 1D to 5Y is representable and round-trips through create/edit UI.
• Engine advancement for value=3, unit=month from 2026-01-31 matches PHP DateInterval math (documented edge: lands on 2026-05-01? No — v1 does single-step: Jan 31 + P3M = May 1st per PHP overflow; assert actual PHP behavior in a pinned test).
• Existing seeded/test rows keep working after migration (backfill map covers all 5 legacy enum values).

Achievable

One migration + form change in Modules/Invoices/Filament/Company/Resources/RecurringInvoices/Schemas/RecurringInvoiceForm.php (Select preset + conditional value/unit inputs) + table column formatter ("Every 2 weeks") + engine increment helper. RecurringInvoicesTest gains cases for custom intervals.

Relevant

Fortnightly and bi-monthly billing are common real-world cadences; this is a hard v1→v2 migration blocker for affected users.

Time-Bound

One sprint; should land before or with the generation engine so the engine's increment helper is written once against the pair.

Arrange

• Schedules: (14, day) a.k.a. fortnightly with next_run_at = today; (3, month) with next_run_at = today; legacy row created pre-migration with enum quarterly.

Act

• Run the migration; run the generation engine.

Assert

• Legacy quarterly row backfilled to frequency_value = 3, frequency_unit = month.
• Fortnightly schedule's next_run_at advanced exactly 14 days; 3-month schedule advanced per PHP DateInterval('P3M') semantics (pinned test).
• Form accepts value 1-99 and unit day/week/month/year; rejects 0 and negatives.
• Table renders "Every 14 days" / "Quarterly" style labels for pairs matching presets.