refactor(api): introduce UserContext schema for API dependency injection

Replace direct User database model imports in API endpoints with UserContext
schema, following the architecture principle that API routes should not import
database models directly.

Changes:
- Create UserContext schema in models/schema/auth.py with from_user() factory
- Update app/api/deps.py to return UserContext from all auth dependencies
- Add _get_user_model() helper for functions needing User model access
- Update 58 API endpoint files to use UserContext instead of User
- Add noqa comments for 4 legitimate edge cases (enums, internal helpers)

Architecture validation: 0 errors (down from 61), 11 warnings remain

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/subscriptions.py

@@ -18,7 +18,7 @@ from app.api.deps import get_current_admin_api
 from app.core.database import get_db
 from app.services.admin_subscription_service import admin_subscription_service
 from app.services.subscription_service import subscription_service
-from models.database.user import User
+from models.schema.auth import UserContext
 from app.modules.billing.schemas import (
     BillingHistoryListResponse,
     BillingHistoryWithVendor,
@@ -46,7 +46,7 @@ logger = logging.getLogger(__name__)
 @router.get("/tiers", response_model=SubscriptionTierListResponse)
 def list_subscription_tiers(
     include_inactive: bool = Query(False, description="Include inactive tiers"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -65,7 +65,7 @@ def list_subscription_tiers(
 @router.get("/tiers/{tier_code}", response_model=SubscriptionTierResponse)
 def get_subscription_tier(
     tier_code: str = Path(..., description="Tier code"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """Get a specific subscription tier by code."""
@@ -76,7 +76,7 @@ def get_subscription_tier(
 @router.post("/tiers", response_model=SubscriptionTierResponse, status_code=201)
 def create_subscription_tier(
     tier_data: SubscriptionTierCreate,
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """Create a new subscription tier."""
@@ -90,7 +90,7 @@ def create_subscription_tier(
 def update_subscription_tier(
     tier_data: SubscriptionTierUpdate,
     tier_code: str = Path(..., description="Tier code"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """Update a subscription tier."""
@@ -104,7 +104,7 @@ def update_subscription_tier(
 @router.delete("/tiers/{tier_code}", status_code=204)
 def delete_subscription_tier(
     tier_code: str = Path(..., description="Tier code"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -128,7 +128,7 @@ def list_vendor_subscriptions(
     status: str | None = Query(None, description="Filter by status"),
     tier: str | None = Query(None, description="Filter by tier"),
     search: str | None = Query(None, description="Search vendor name"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -165,7 +165,7 @@ def list_vendor_subscriptions(
 
 @router.get("/stats", response_model=SubscriptionStatsResponse)
 def get_subscription_stats(
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """Get subscription statistics for admin dashboard."""
@@ -184,7 +184,7 @@ def list_billing_history(
     per_page: int = Query(20, ge=1, le=100),
     vendor_id: int | None = Query(None, description="Filter by vendor"),
     status: str | None = Query(None, description="Filter by status"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """List billing history (invoices) across all vendors."""
@@ -234,7 +234,7 @@ def list_billing_history(
 def create_vendor_subscription(
     create_data: VendorSubscriptionCreate,
     vendor_id: int = Path(..., description="Vendor ID"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -280,7 +280,7 @@ def create_vendor_subscription(
 @router.get("/{vendor_id}", response_model=VendorSubscriptionWithVendor)
 def get_vendor_subscription(
     vendor_id: int = Path(..., description="Vendor ID"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """Get subscription details for a specific vendor."""
@@ -302,7 +302,7 @@ def get_vendor_subscription(
 def update_vendor_subscription(
     update_data: VendorSubscriptionUpdate,
     vendor_id: int = Path(..., description="Vendor ID"),
-    current_user: User = Depends(get_current_admin_api),
+    current_user: UserContext = Depends(get_current_admin_api),
     db: Session = Depends(get_db),
 ):
     """