feat(loyalty): expiry settings for both expiry models

Settles note N4 of docs/prd-point-coin.md: both expiry models are
supported, chosen per currency by the owner, defaulting to one fixed date a
year (PC-501, F12).

New organization keys, per currency (loyalty.point.* / loyalty.coin.*):
- expiry_mode: FIXED_DATE (default) or ROLLING.
- expiry_fixed_dates: the days of the year balances expire on, as sorted
  MM-DD values ("12-31" by default, "06-30,12-31" for twice a year). 29 Feb
  is refused.
- expiry_grace_months: 0 to 24, default 3. A balance lasts at least this
  long before a fixed date takes it.
The existing period, unit and end_of_month keys now belong to ROLLING, and
reminder_days to both.

ComputeExpiry gives the expiry of a balance received at a time: the first
fixed date on or after the day received plus the grace months, or the day
received plus the period (to the end of that month when asked). Days are
the customer's (WIB), a shorter month keeps to its last day, and a lot
lasts to 23:59:59 of its day so the apps group it under that day. Nil when
expiry is off. ActivationExpiry, RefundExpiry and EarlierExpiry hold the
other decided rules and are used by PC-502.

GET and PUT /marketing/loyalty-settings return expiry_preview: when a
balance received now would expire, for the dashboard's "received today
expires on ..." hint, also on a dry run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/models/loyalty.go

@@ -57,15 +57,29 @@ type LoyaltyTransferSettings struct {
 	DailyLimit        *int64 `json:"daily_limit"`
 }
 
-// LoyaltyExpirySettings is how long one currency lasts once received. The expiry
-// model is still open (note N4); these are only the stored settings.
+// LoyaltyExpirySettings is when one currency expires once received (F12). Both
+// models of note N4 are supported, and the owner picks one:
+//
+//   - FIXED_DATE: everything expires on the next of FixedDates falling on or after
+//     the day received + GraceMonths, so a balance received just before a date moves
+//     on to the one after.
+//   - ROLLING: everything lasts Period Units from the day received, to the end of
+//     that month when EndOfMonth is set.
 type LoyaltyExpirySettings struct {
-	Enabled bool  `json:"enabled"`
-	Period  int64 `json:"period"`
-	// DAY or MONTH.
-	Unit         string `json:"unit"`
-	EndOfMonth   bool   `json:"end_of_month"`
-	ReminderDays int64  `json:"reminder_days"`
+	Enabled bool `json:"enabled"`
+	// FIXED_DATE or ROLLING.
+	Mode string `json:"mode"`
+	// FIXED_DATE: the days of the year balances expire on, as MM-DD, sorted.
+	FixedDates []string `json:"fixed_dates"`
+	// FIXED_DATE: how many months a balance lasts at least before a fixed date takes it.
+	GraceMonths int64 `json:"grace_months"`
+	// ROLLING: how long a balance lasts.
+	Period int64 `json:"period"`
+	// ROLLING: DAY or MONTH.
+	Unit       string `json:"unit"`
+	EndOfMonth bool   `json:"end_of_month"`
+	// Days before expiry the customer is reminded; 0 for no reminder.
+	ReminderDays int64 `json:"reminder_days"`
 }
 
 // LoyaltySettingChange is one row of the loyalty settings history.
@@ -109,12 +123,21 @@ type OrganizationLoyaltySettingsView struct {
 	OrganizationLoyaltySettings
 	// What the balances in circulation are worth, before and after the change.
 	Impact LoyaltySettingsImpact `json:"impact"`
+	// When a balance received now would expire under these settings (F12).
+	ExpiryPreview LoyaltyExpiryPreview `json:"expiry_preview"`
 	// On PUT, the keys that changed; on a dry run, the keys that would.
 	Changes []LoyaltySettingChange `json:"changes"`
 	// True when nothing was saved.
 	DryRun bool `json:"dry_run"`
 }
 
+// LoyaltyExpiryPreview is what the dashboard shows next to the expiry settings: "the
+// EnakPoint received today expire on …". Nil means they never expire.
+type LoyaltyExpiryPreview struct {
+	Point *time.Time `json:"point"`
+	Coin  *time.Time `json:"coin"`
+}
+
 // LoyaltySettingsImpact shows how a change of point value or exchange rate changes what
 // the balances in circulation are worth (F2). Before and after are equal when neither
 // changes.