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_provider_token.py

@@ -0,0 +1,146 @@
+# app/repositories/oauth_provider_token.py
+"""Repository for OAuthProviderRefreshToken model."""
+
+import logging
+from datetime import UTC, datetime, timedelta
+from uuid import UUID
+
+from sqlalchemy import and_, delete, select, update
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.oauth_provider_token import OAuthProviderRefreshToken
+
+logger = logging.getLogger(__name__)
+
+
+class OAuthProviderTokenRepository:
+    """Repository for OAuth provider refresh tokens."""
+
+    async def create_token(
+        self,
+        db: AsyncSession,
+        *,
+        token_hash: str,
+        jti: str,
+        client_id: str,
+        user_id: UUID,
+        scope: str,
+        expires_at: datetime,
+        device_info: str | None = None,
+        ip_address: str | None = None,
+    ) -> OAuthProviderRefreshToken:
+        """Create and persist a new refresh token record."""
+        token = OAuthProviderRefreshToken(
+            token_hash=token_hash,
+            jti=jti,
+            client_id=client_id,
+            user_id=user_id,
+            scope=scope,
+            expires_at=expires_at,
+            device_info=device_info,
+            ip_address=ip_address,
+        )
+        db.add(token)
+        await db.commit()
+        return token
+
+    async def get_by_token_hash(
+        self, db: AsyncSession, *, token_hash: str
+    ) -> OAuthProviderRefreshToken | None:
+        """Get refresh token record by SHA-256 token hash."""
+        result = await db.execute(
+            select(OAuthProviderRefreshToken).where(
+                OAuthProviderRefreshToken.token_hash == token_hash
+            )
+        )
+        return result.scalar_one_or_none()
+
+    async def get_by_jti(
+        self, db: AsyncSession, *, jti: str
+    ) -> OAuthProviderRefreshToken | None:
+        """Get refresh token record by JWT ID (JTI)."""
+        result = await db.execute(
+            select(OAuthProviderRefreshToken).where(
+                OAuthProviderRefreshToken.jti == jti
+            )
+        )
+        return result.scalar_one_or_none()
+
+    async def revoke(
+        self, db: AsyncSession, *, token: OAuthProviderRefreshToken
+    ) -> None:
+        """Mark a specific token record as revoked."""
+        token.revoked = True  # type: ignore[assignment]
+        token.last_used_at = datetime.now(UTC)  # type: ignore[assignment]
+        await db.commit()
+
+    async def revoke_all_for_user_client(
+        self, db: AsyncSession, *, user_id: UUID, client_id: str
+    ) -> int:
+        """
+        Revoke all active tokens for a specific user-client pair.
+
+        Used when security incidents are detected (e.g., authorization code reuse).
+        Returns the number of tokens revoked.
+        """
+        result = await db.execute(
+            update(OAuthProviderRefreshToken)
+            .where(
+                and_(
+                    OAuthProviderRefreshToken.user_id == user_id,
+                    OAuthProviderRefreshToken.client_id == client_id,
+                    OAuthProviderRefreshToken.revoked == False,  # noqa: E712
+                )
+            )
+            .values(revoked=True)
+        )
+        count = result.rowcount  # type: ignore[attr-defined]
+        if count > 0:
+            await db.commit()
+        return count
+
+    async def revoke_all_for_user(
+        self, db: AsyncSession, *, user_id: UUID
+    ) -> int:
+        """
+        Revoke all active tokens for a user across all clients.
+
+        Used when user changes password or logs out everywhere.
+        Returns the number of tokens revoked.
+        """
+        result = await db.execute(
+            update(OAuthProviderRefreshToken)
+            .where(
+                and_(
+                    OAuthProviderRefreshToken.user_id == user_id,
+                    OAuthProviderRefreshToken.revoked == False,  # noqa: E712
+                )
+            )
+            .values(revoked=True)
+        )
+        count = result.rowcount  # type: ignore[attr-defined]
+        if count > 0:
+            await db.commit()
+        return count
+
+    async def cleanup_expired(
+        self, db: AsyncSession, *, cutoff_days: int = 7
+    ) -> int:
+        """
+        Delete expired refresh tokens older than cutoff_days.
+
+        Should be called periodically (e.g., daily).
+        Returns the number of tokens deleted.
+        """
+        cutoff = datetime.now(UTC) - timedelta(days=cutoff_days)
+        result = await db.execute(
+            delete(OAuthProviderRefreshToken).where(
+                OAuthProviderRefreshToken.expires_at < cutoff
+            )
+        )
+        await db.commit()
+        return result.rowcount  # type: ignore[attr-defined]
+
+
+# Singleton instance
+oauth_provider_token_repo = OAuthProviderTokenRepository()