Add token revocation mechanism and support for logout APIs

This commit introduces a system to revoke tokens by storing their `jti` in a new `RevokedToken` model. It includes APIs for logging out (revoking a current token) and logging out from all devices (revoking all tokens). Additionally, token validation now checks revocation status during the decode process.

backend/app/models/token.py

@@ -0,0 +1,15 @@
+from sqlalchemy import Column, String, ForeignKey
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship
+from app.models.base import Base, TimestampMixin, UUIDMixin
+
+
+class RevokedToken(UUIDMixin, TimestampMixin, Base):
+    """Model to store revoked JWT tokens via their jti (JWT ID)."""
+    __tablename__ = "revoked_tokens"
+
+    jti = Column(String(length=50), nullable=False, unique=True, index=True)
+    token_type = Column(String(length=20), nullable=False)
+    user_id = Column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="CASCADE"))
+
+    user = relationship("User", back_populates="revoked_tokens")