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/repositories/oauth_consent.py

@@ -0,0 +1,112 @@
+# app/repositories/oauth_consent.py
+"""Repository for OAuthConsent model."""
+
+import logging
+from uuid import UUID
+
+from typing import Any
+
+from sqlalchemy import and_, delete, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.oauth_client import OAuthClient
+from app.models.oauth_provider_token import OAuthConsent
+
+logger = logging.getLogger(__name__)
+
+
+class OAuthConsentRepository:
+    """Repository for OAuth consent records (user grants to clients)."""
+
+    async def get_consent(
+        self, db: AsyncSession, *, user_id: UUID, client_id: str
+    ) -> OAuthConsent | None:
+        """Get the consent record for a user-client pair, or None if not found."""
+        result = await db.execute(
+            select(OAuthConsent).where(
+                and_(
+                    OAuthConsent.user_id == user_id,
+                    OAuthConsent.client_id == client_id,
+                )
+            )
+        )
+        return result.scalar_one_or_none()
+
+    async def grant_consent(
+        self,
+        db: AsyncSession,
+        *,
+        user_id: UUID,
+        client_id: str,
+        scopes: list[str],
+    ) -> OAuthConsent:
+        """
+        Create or update consent for a user-client pair.
+
+        If consent already exists, the new scopes are merged with existing ones.
+        Returns the created or updated consent record.
+        """
+        consent = await self.get_consent(db, user_id=user_id, client_id=client_id)
+
+        if consent:
+            existing = set(consent.granted_scopes.split()) if consent.granted_scopes else set()
+            merged = existing | set(scopes)
+            consent.granted_scopes = " ".join(sorted(merged))  # type: ignore[assignment]
+        else:
+            consent = OAuthConsent(
+                user_id=user_id,
+                client_id=client_id,
+                granted_scopes=" ".join(sorted(set(scopes))),
+            )
+            db.add(consent)
+
+        await db.commit()
+        await db.refresh(consent)
+        return consent
+
+    async def get_user_consents_with_clients(
+        self, db: AsyncSession, *, user_id: UUID
+    ) -> list[dict[str, Any]]:
+        """Get all consent records for a user joined with client details."""
+        result = await db.execute(
+            select(OAuthConsent, OAuthClient)
+            .join(OAuthClient, OAuthConsent.client_id == OAuthClient.client_id)
+            .where(OAuthConsent.user_id == user_id)
+        )
+        rows = result.all()
+        return [
+            {
+                "client_id": consent.client_id,
+                "client_name": client.client_name,
+                "client_description": client.client_description,
+                "granted_scopes": consent.granted_scopes.split()
+                if consent.granted_scopes
+                else [],
+                "granted_at": consent.created_at.isoformat(),
+            }
+            for consent, client in rows
+        ]
+
+    async def revoke_consent(
+        self, db: AsyncSession, *, user_id: UUID, client_id: str
+    ) -> bool:
+        """
+        Delete the consent record for a user-client pair.
+
+        Returns True if a record was found and deleted.
+        Note: Callers are responsible for also revoking associated tokens.
+        """
+        result = await db.execute(
+            delete(OAuthConsent).where(
+                and_(
+                    OAuthConsent.user_id == user_id,
+                    OAuthConsent.client_id == client_id,
+                )
+            )
+        )
+        await db.commit()
+        return result.rowcount > 0  # type: ignore[attr-defined]
+
+
+# Singleton instance
+oauth_consent_repo = OAuthConsentRepository()