refactor(backend): enforce route→service→repo layered architecture

- introduce custom repository exception hierarchy (DuplicateEntryError,
  IntegrityConstraintError, InvalidInputError) replacing raw ValueError
- eliminate all direct repository imports and raw SQL from route layer
- add UserService, SessionService, OrganizationService to service layer
- add get_stats/get_org_distribution service methods replacing admin inline SQL
- fix timing side-channel in authenticate_user via dummy bcrypt check
- replace SHA-256 client secret fallback with explicit InvalidClientError
- replace assert with InvalidGrantError in authorization code exchange
- replace N+1 token revocation loops with bulk UPDATE statements
- rename oauth account token fields (drop misleading 'encrypted' suffix)
- add Alembic migration 0003 for token field column rename
- add 45 new service/repository tests; 975 passing, 94% coverage

backend/app/api/dependencies/auth.py

@@ -1,12 +1,12 @@
 from fastapi import Depends, Header, HTTPException, status
 from fastapi.security import OAuth2PasswordBearer
 from fastapi.security.utils import get_authorization_scheme_param
-from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.auth import TokenExpiredError, TokenInvalidError, get_token_data
 from app.core.database import get_db
 from app.models.user import User
+from app.repositories.user import user_repo
 
 # OAuth2 configuration
 oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/v1/auth/login")
@@ -32,9 +32,8 @@ async def get_current_user(
         # Decode token and get user ID
         token_data = get_token_data(token)
 
-        # Get user from database
-        result = await db.execute(select(User).where(User.id == token_data.user_id))
-        user = result.scalar_one_or_none()
+        # Get user from database via repository
+        user = await user_repo.get(db, id=str(token_data.user_id))
 
         if not user:
             raise HTTPException(
@@ -144,8 +143,7 @@ async def get_optional_current_user(
 
     try:
         token_data = get_token_data(token)
-        result = await db.execute(select(User).where(User.id == token_data.user_id))
-        user = result.scalar_one_or_none()
+        user = await user_repo.get(db, id=str(token_data.user_id))
         if not user or not user.is_active:
             return None
         return user

backend/app/api/dependencies/permissions.py

@@ -15,9 +15,9 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.dependencies.auth import get_current_user
 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
+from app.services.organization_service import organization_service
 
 
 def require_superuser(current_user: User = Depends(get_current_user)) -> User:
@@ -81,7 +81,7 @@ class OrganizationPermission:
             return current_user
 
         # Get user's role in organization
-        user_role = await organization_crud.get_user_role_in_org(
+        user_role = await organization_service.get_user_role_in_org(
             db, user_id=current_user.id, organization_id=organization_id
         )
 
@@ -123,7 +123,7 @@ async def require_org_membership(
     if current_user.is_superuser:
         return current_user
 
-    user_role = await organization_crud.get_user_role_in_org(
+    user_role = await organization_service.get_user_role_in_org(
         db, user_id=current_user.id, organization_id=organization_id
     )
 

backend/app/api/dependencies/services.py

@@ -0,0 +1,41 @@
+# app/api/dependencies/services.py
+"""FastAPI dependency functions for service singletons."""
+
+from app.services import oauth_provider_service
+from app.services.auth_service import AuthService
+from app.services.oauth_service import OAuthService
+from app.services.organization_service import OrganizationService, organization_service
+from app.services.session_service import SessionService, session_service
+from app.services.user_service import UserService, user_service
+
+
+def get_auth_service() -> AuthService:
+    """Return the AuthService singleton for dependency injection."""
+    from app.services.auth_service import AuthService as _AuthService
+
+    return _AuthService()
+
+
+def get_user_service() -> UserService:
+    """Return the UserService singleton for dependency injection."""
+    return user_service
+
+
+def get_organization_service() -> OrganizationService:
+    """Return the OrganizationService singleton for dependency injection."""
+    return organization_service
+
+
+def get_session_service() -> SessionService:
+    """Return the SessionService singleton for dependency injection."""
+    return session_service
+
+
+def get_oauth_service() -> OAuthService:
+    """Return OAuthService for dependency injection."""
+    return OAuthService()
+
+
+def get_oauth_provider_service():
+    """Return the oauth_provider_service module for dependency injection."""
+    return oauth_provider_service