feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/service/payment_config_limits.go

@@ -0,0 +1,172 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	dbent "github.com/Wei-Shaw/sub2api/ent"
+	"github.com/Wei-Shaw/sub2api/ent/paymentproviderinstance"
+	"github.com/Wei-Shaw/sub2api/internal/payment"
+)
+
+// GetAvailableMethodLimits collects all payment types from enabled provider
+// instances and returns limits for each, plus the global widest range.
+// Stripe sub-types (card, link) are aggregated under "stripe".
+func (s *PaymentConfigService) GetAvailableMethodLimits(ctx context.Context) (*MethodLimitsResponse, error) {
+	instances, err := s.entClient.PaymentProviderInstance.Query().
+		Where(paymentproviderinstance.EnabledEQ(true)).All(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("query provider instances: %w", err)
+	}
+	typeInstances := pcGroupByPaymentType(instances)
+	resp := &MethodLimitsResponse{
+		Methods: make(map[string]MethodLimits, len(typeInstances)),
+	}
+	for pt, insts := range typeInstances {
+		ml := pcAggregateMethodLimits(pt, insts)
+		resp.Methods[ml.PaymentType] = ml
+	}
+	resp.GlobalMin, resp.GlobalMax = pcComputeGlobalRange(resp.Methods)
+	return resp, nil
+}
+
+// GetMethodLimits returns per-payment-type limits from enabled provider instances.
+func (s *PaymentConfigService) GetMethodLimits(ctx context.Context, types []string) ([]MethodLimits, error) {
+	instances, err := s.entClient.PaymentProviderInstance.Query().
+		Where(paymentproviderinstance.EnabledEQ(true)).All(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("query provider instances: %w", err)
+	}
+	result := make([]MethodLimits, 0, len(types))
+	for _, pt := range types {
+		var matching []*dbent.PaymentProviderInstance
+		for _, inst := range instances {
+			if payment.InstanceSupportsType(inst.SupportedTypes, pt) {
+				matching = append(matching, inst)
+			}
+		}
+		result = append(result, pcAggregateMethodLimits(pt, matching))
+	}
+	return result, nil
+}
+
+// pcGroupByPaymentType groups instances by user-facing payment type.
+// For Stripe providers, ALL sub-types (card, link, alipay, wxpay) map to "stripe"
+// because the user sees a single "Stripe" button, not individual sub-methods.
+// Uses a seen set to avoid counting one instance twice.
+func pcGroupByPaymentType(instances []*dbent.PaymentProviderInstance) map[string][]*dbent.PaymentProviderInstance {
+	typeInstances := make(map[string][]*dbent.PaymentProviderInstance)
+	seen := make(map[string]map[int64]bool)
+	add := func(key string, inst *dbent.PaymentProviderInstance) {
+		if seen[key] == nil {
+			seen[key] = make(map[int64]bool)
+		}
+		if !seen[key][int64(inst.ID)] {
+			seen[key][int64(inst.ID)] = true
+			typeInstances[key] = append(typeInstances[key], inst)
+		}
+	}
+	for _, inst := range instances {
+		// Stripe provider: all sub-types → single "stripe" group
+		if inst.ProviderKey == payment.TypeStripe {
+			add(payment.TypeStripe, inst)
+			continue
+		}
+		for _, t := range splitTypes(inst.SupportedTypes) {
+			add(t, inst)
+		}
+	}
+	return typeInstances
+}
+
+// pcInstanceTypeLimits extracts per-type limits from a provider instance.
+// Returns (limits, true) if configured; (zero, false) if unlimited.
+// For Stripe instances, limits are stored under "stripe" key regardless of sub-types.
+func pcInstanceTypeLimits(inst *dbent.PaymentProviderInstance, pt string) (payment.ChannelLimits, bool) {
+	if inst.Limits == "" {
+		return payment.ChannelLimits{}, false
+	}
+	var limits payment.InstanceLimits
+	if err := json.Unmarshal([]byte(inst.Limits), &limits); err != nil {
+		return payment.ChannelLimits{}, false
+	}
+	cl, ok := limits[pt]
+	return cl, ok
+}
+
+// unionFloat merges a single limit value into the aggregate using UNION semantics.
+//   - For "min" fields (wantMin=true): keeps the lowest non-zero value
+//   - For "max"/"cap" fields (wantMin=false): keeps the highest non-zero value
+//   - If any value is 0 (unlimited), the result is unlimited.
+//
+// Returns (aggregated value, still limited).
+func unionFloat(agg float64, limited bool, val float64, wantMin bool) (float64, bool) {
+	if val == 0 {
+		return agg, false
+	}
+	if !limited {
+		return agg, false
+	}
+	if agg == 0 {
+		return val, true
+	}
+	if wantMin && val < agg {
+		return val, true
+	}
+	if !wantMin && val > agg {
+		return val, true
+	}
+	return agg, true
+}
+
+// pcAggregateMethodLimits computes the UNION (least restrictive) of limits
+// across all provider instances for a given payment type.
+//
+// Since the load balancer can route an order to any available instance,
+// the user should see the widest possible range:
+//   - SingleMin: lowest floor across instances; 0 if any is unlimited
+//   - SingleMax: highest ceiling across instances; 0 if any is unlimited
+//   - DailyLimit: highest cap across instances; 0 if any is unlimited
+func pcAggregateMethodLimits(pt string, instances []*dbent.PaymentProviderInstance) MethodLimits {
+	ml := MethodLimits{PaymentType: pt}
+	minLimited, maxLimited, dailyLimited := true, true, true
+
+	for _, inst := range instances {
+		cl, hasLimits := pcInstanceTypeLimits(inst, pt)
+		if !hasLimits {
+			return MethodLimits{PaymentType: pt} // any unlimited instance → all zeros
+		}
+		ml.SingleMin, minLimited = unionFloat(ml.SingleMin, minLimited, cl.SingleMin, true)
+		ml.SingleMax, maxLimited = unionFloat(ml.SingleMax, maxLimited, cl.SingleMax, false)
+		ml.DailyLimit, dailyLimited = unionFloat(ml.DailyLimit, dailyLimited, cl.DailyLimit, false)
+	}
+
+	if !minLimited {
+		ml.SingleMin = 0
+	}
+	if !maxLimited {
+		ml.SingleMax = 0
+	}
+	if !dailyLimited {
+		ml.DailyLimit = 0
+	}
+	return ml
+}
+
+// pcComputeGlobalRange computes the widest [min, max] across all methods.
+// Uses the same union logic: lowest min, highest max, 0 if any is unlimited.
+func pcComputeGlobalRange(methods map[string]MethodLimits) (globalMin, globalMax float64) {
+	minLimited, maxLimited := true, true
+	for _, ml := range methods {
+		globalMin, minLimited = unionFloat(globalMin, minLimited, ml.SingleMin, true)
+		globalMax, maxLimited = unionFloat(globalMax, maxLimited, ml.SingleMax, false)
+	}
+	if !minLimited {
+		globalMin = 0
+	}
+	if !maxLimited {
+		globalMax = 0
+	}
+	return globalMin, globalMax
+}