feat(api-key): add independent quota and expiration support

This feature allows API Keys to have their own quota limits and expiration
times, independent of the user's balance.

Backend:
- Add quota, quota_used, expires_at fields to api_key schema
- Implement IsExpired() and IsQuotaExhausted() checks in middleware
- Add ResetQuota and ClearExpiration API endpoints
- Integrate quota billing in gateway handlers (OpenAI, Anthropic, Gemini)
- Include quota/expiration fields in auth cache for performance
- Expiration check returns 403, quota exhausted returns 429

Frontend:
- Add quota and expiration inputs to key create/edit dialog
- Add quick-select buttons for expiration (+7, +30, +90 days)
- Add reset quota confirmation dialog
- Add expires_at column to keys list
- Add i18n translations for new features (en/zh)

Migration:
- Add 045_add_api_key_quota.sql for new columns

backend/ent/schema/api_key.go

@@ -5,6 +5,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/domain"
 
 	"entgo.io/ent"
+	"entgo.io/ent/dialect"
 	"entgo.io/ent/dialect/entsql"
 	"entgo.io/ent/schema"
 	"entgo.io/ent/schema/edge"
@@ -52,6 +53,23 @@ func (APIKey) Fields() []ent.Field {
 		field.JSON("ip_blacklist", []string{}).
 			Optional().
 			Comment("Blocked IPs/CIDRs"),
+
+		// ========== Quota fields ==========
+		// Quota limit in USD (0 = unlimited)
+		field.Float("quota").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
+			Default(0).
+			Comment("Quota limit in USD for this API key (0 = unlimited)"),
+		// Used quota amount
+		field.Float("quota_used").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
+			Default(0).
+			Comment("Used quota amount in USD"),
+		// Expiration time (nil = never expires)
+		field.Time("expires_at").
+			Optional().
+			Nillable().
+			Comment("Expiration time for this API key (null = never expires)"),
 	}
 }
 
@@ -77,5 +95,8 @@ func (APIKey) Indexes() []ent.Index {
 		index.Fields("group_id"),
 		index.Fields("status"),
 		index.Fields("deleted_at"),
+		// Index for quota queries
+		index.Fields("quota", "quota_used"),
+		index.Fields("expires_at"),
 	}
 }