feat(auth): jti claim and token-revocation store

Stateless JWTs had no revocation path: a stolen token stayed valid for
its full 24h even after the victim changed their password, and there was
no logout. This lays the foundation for revoking them.

- User.tokens_valid_from: per-user bulk-revocation cutoff (compared against
  the token's iat). RevokedToken(jti PK, exp): single-token denylist, pruned
  opportunistically on insert so it never outgrows live-but-revoked tokens.
- login() now mints a jti; create_access_token already stamps iat/exp.
- repo.revoke_token / is_token_revoked / set_tokens_valid_from (abstract +
  shared sqlmodel impl + DummyRepo coverage stubs).
- Centralized validate path in dependencies.py: every auth dependency now
  resolves the user and fails closed on (1) missing jti (legacy/pre-deploy
  token -> one forced re-login), (2) iat before the cutoff, (3) a denylisted
  jti. Denylist lookups ride a 10s membership cache mirroring the user cache.
- Contract/fuzz harness seeds its fixed-uuid principal under
  DECNET_CONTRACT_TEST so its minted token resolves to a live admin user.

decnet/web/db/models/__init__.py

@@ -38,6 +38,7 @@ from .auth import (
     GlobalMutationIntervalRequest,
     LoginRequest,
     ResetUserPasswordRequest,
+    RevokedToken,
     Token,
     UpdateUserRoleRequest,
     User,
@@ -254,6 +255,7 @@ __all__ = [
     "GlobalMutationIntervalRequest",
     "LoginRequest",
     "ResetUserPasswordRequest",
+    "RevokedToken",
     "Token",
     "UpdateUserRoleRequest",
     "User",