feat(billing): add GET /billing/state aggregating subscription + plan + features

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/schemas/billing.py

@@ -1,4 +1,5 @@
-from typing import Literal
+from typing import Literal, Optional, Dict, Any
+from datetime import datetime
 from pydantic import BaseModel
 
 
@@ -10,3 +11,30 @@ class CheckoutSessionCreate(BaseModel):
 
 class CheckoutSessionResponse(BaseModel):
     url: str
+
+
+class SubscriptionState(BaseModel):
+    status: str
+    plan: str
+    current_period_start: Optional[datetime]
+    current_period_end: Optional[datetime]
+    cancel_at_period_end: bool
+    seat_limit: Optional[int]
+    has_pro_entitlement: bool
+    is_paid: bool
+
+
+class PlanBillingState(BaseModel):
+    display_name: str
+    description: Optional[str] = None
+    monthly_price_cents: Optional[int] = None
+    annual_price_cents: Optional[int] = None
+
+    model_config = {"from_attributes": True}
+
+
+class BillingStateResponse(BaseModel):
+    subscription: SubscriptionState
+    plan_billing: Optional[PlanBillingState]
+    plan_limits: Dict[str, Any]
+    enabled_features: Dict[str, bool]