Remove unused async database and CRUD modules

- Deleted `database_async.py`, `base_async.py`, and `organization_async.py` modules due to deprecation and unused references across the project.
- Improved overall codebase clarity and minimized redundant functionality by removing unused async database logic, CRUD utilities, and organization-related operations.

backend/app/api/dependencies/auth.py

@@ -7,7 +7,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.auth import get_token_data, TokenExpiredError, TokenInvalidError
-from app.core.database_async import get_async_db
+from app.core.database import get_db
 from app.models.user import User
 
 # OAuth2 configuration
@@ -15,7 +15,7 @@ oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/v1/auth/login")
 
 
 async def get_current_user(
-        db: AsyncSession = Depends(get_async_db),
+        db: AsyncSession = Depends(get_db),
         token: str = Depends(oauth2_scheme)
 ) -> User:
     """
@@ -139,7 +139,7 @@ async def get_optional_token(authorization: str = Header(None)) -> Optional[str]
 
 
 async def get_optional_current_user(
-        db: AsyncSession = Depends(get_async_db),
+        db: AsyncSession = Depends(get_db),
         token: Optional[str] = Depends(get_optional_token)
 ) -> Optional[User]:
     """

backend/app/api/dependencies/permissions.py

@@ -14,8 +14,8 @@ from fastapi import Depends, HTTPException, status
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.dependencies.auth import get_current_user
-from app.core.database_async import get_async_db
-from app.crud.organization_async import organization_async as organization_crud
+from app.core.database import get_db
+from app.crud.organization import organization as organization_crud
 from app.models.user import User
 from app.models.user_organization import OrganizationRole
 
@@ -78,7 +78,7 @@ class OrganizationPermission:
         self,
         organization_id: UUID,
         current_user: User = Depends(get_current_user),
-        db: AsyncSession = Depends(get_async_db)
+        db: AsyncSession = Depends(get_db)
     ) -> User:
         """
         Check if user has required role in the organization.
@@ -133,7 +133,7 @@ require_org_member = OrganizationPermission([
 async def get_current_org_role(
     organization_id: UUID,
     current_user: User = Depends(get_current_user),
-    db: AsyncSession = Depends(get_async_db)
+    db: AsyncSession = Depends(get_db)
 ) -> Optional[OrganizationRole]:
     """
     Get the current user's role in an organization.
@@ -164,7 +164,7 @@ async def get_current_org_role(
 async def require_org_membership(
     organization_id: UUID,
     current_user: User = Depends(get_current_user),
-    db: AsyncSession = Depends(get_async_db)
+    db: AsyncSession = Depends(get_db)
 ) -> User:
     """
     Ensure user is a member of the organization (any role).