refactor(monitor): remove deprecated challenge flow

- Remove challenge issuance endpoint
- Remove monitor websocket challenge handshake flow
- Remove challenge/nonce runtime models
- Keep API key as the only server auth path

app/models/monitor.py

@@ -59,22 +59,3 @@ class ServerState(Base):
     last_seen_at = Column(DateTime(timezone=True), nullable=True)
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
 
-
-class ServerChallenge(Base):
-    __tablename__ = 'server_challenges'
-
-    id = Column(Integer, primary_key=True, index=True)
-    server_id = Column(Integer, ForeignKey('monitored_servers.id'), nullable=False, index=True)
-    challenge_uuid = Column(String(64), nullable=False, unique=True, index=True)
-    expires_at = Column(DateTime(timezone=True), nullable=False)
-    used_at = Column(DateTime(timezone=True), nullable=True)
-    created_at = Column(DateTime(timezone=True), server_default=func.now())
-
-
-class ServerHandshakeNonce(Base):
-    __tablename__ = 'server_handshake_nonces'
-
-    id = Column(Integer, primary_key=True, index=True)
-    server_id = Column(Integer, ForeignKey('monitored_servers.id'), nullable=False, index=True)
-    nonce = Column(String(128), nullable=False, index=True)
-    created_at = Column(DateTime(timezone=True), server_default=func.now())