Add bulk grant permission API endpoint

New Features:
- BulkGrantPermission model: Grant same permission to multiple users
- BulkGrantResult model: Detailed success/failure results
- POST /api/v1/admin/permissions/bulk-grant endpoint

This simplifies the admin workflow for granting the same account
permission to multiple users at once (e.g., onboarding a team).

The endpoint validates the account exists and is active, then grants
the permission to each user, collecting successes and failures to
return a detailed result.

Related: UI-IMPROVEMENTS-PLAN.md Phase 1

models.py

@@ -315,6 +315,24 @@ class CreateAccountPermission(BaseModel):
     notes: Optional[str] = None
 
 
+class BulkGrantPermission(BaseModel):
+    """Bulk grant same permission to multiple users"""
+    user_ids: list[str]  # List of user IDs to grant permission to
+    account_id: str  # Account to grant permission on
+    permission_type: PermissionType  # Type of permission to grant
+    expires_at: Optional[datetime] = None  # Optional expiration
+    notes: Optional[str] = None  # Notes for all permissions
+
+
+class BulkGrantResult(BaseModel):
+    """Result of bulk grant operation"""
+    granted: list[AccountPermission]  # Successfully granted permissions
+    failed: list[dict]  # Failed grants with errors
+    total: int  # Total attempted
+    success_count: int  # Number of successful grants
+    failure_count: int  # Number of failed grants
+
+
 class AccountWithPermissions(BaseModel):
     """Account with user-specific permission metadata"""
     id: str