Every bearer-authenticated request, including each node-sync call, loaded all
enabled api_tokens rows and compared each stored SHA-256 hash in constant time.
Tokens are stored hashed, so the database can compare hashes directly: a
timing leak there reveals only a stored hash, which cannot authenticate. The
lookup is now one indexed seek on idx_api_tokens_token, which AutoMigrate
creates from the model tag on new and existing databases; scope and expiry
are still checked afterwards.