Add weekly and monthly scheduled billing for offerings

Offerings can now bill weekly (a pending payment 24h before each lesson)
or monthly (one payment on the 1st for that month's lessons), alongside
one-time and full-term. Applies to both private lessons and group classes.

- Offering: new `weekly`/`monthly` billing modes + `isScheduledBilling()`
- Booking/enrolment defer payment for scheduled modes; a single lesson
  booked after its due date has passed (e.g. an add-on in an already-billed
  month) is charged at booking instead
- ScheduledBillingRunner: daily WP-Cron scan generates due payments across
  four cases (private/group × weekly/monthly), deduped via lesson.payment_id
  and payments.period_key
- PaymentDueMailer: one consolidated itemised email per student per scan
- Notice batch: payments emailed together share a reference; the admin
  Payments queue groups them with a lump-sum total for e-transfer reconciliation
- Cancellation never voids a scheduled payment (Payment::isScheduled())
- Schema: us_payments gains due_date, period_key, notice_batch; USC_VERSION 1.2.0

composer test, composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

docs/features/offerings.md

@@ -15,7 +15,7 @@ An offering is anything a student can register for: a private-lesson type (30 or
 | `duration_minutes` | SMALLINT         | Private lessons only (e.g. 30, 60); NULL for group classes  |
 | `price`            | DECIMAL(10,2)    | Price in dollars                                            |
 | `currency`         | VARCHAR(3)       | ISO 4217, e.g. `CAD`                                        |
-| `billing_mode`     | VARCHAR(20)      | `one_time` (single booking) or `full_term` (weekly / group) |
+| `billing_mode`     | VARCHAR(20)      | `one_time`, `full_term`, `weekly`, or `monthly` (see Billing Mode below) |
 | `allow_weekly`     | TINYINT(1)       | Private only — may be reserved weekly for the term          |
 | `capacity`         | SMALLINT         | Group only — max enrolments; NULL for private               |
 | `term_start`       | DATE             | Group / term offerings — first day; NULL otherwise          |
@@ -31,6 +31,12 @@ An offering is anything a student can register for: a private-lesson type (30 or
 ## Billing Mode
 - `one_time` — charged once at booking (a single private lesson).
 - `full_term` — charged in full upfront at registration (a weekly private reservation or a year-long group class). See `payments.md`.
+- `weekly` — **not** charged at registration; a pending payment for one lesson's fee is generated **24 hours before each lesson** by the daily billing scan.
+- `monthly` — **not** charged at registration; on the **1st of each month** a single pending payment is generated for every lesson that falls in that month (4 lessons ⇒ 4 × fee).
+
+`weekly` and `monthly` are *scheduled* billing (`Offering::isScheduledBilling()`): the
+booking/enrolment succeeds with no payment step, and payments are created later by the
+daily `us_generate_due_payments` cron scan. See `scheduled-billing.md` and `payments.md`.
 
 ## Term Dates
 Group classes carry a term: `term_start` is the date of the first class and