feat(jwt): server-side session table with keyId + obfuscated uid claims

Replace the {user_id, ip} claim pair with {keyId, uid}:
- keyId is a 32-byte random id that points to a row in the new
  jwt_sessions table holding the real user id, bound IP, UA hash,
  TokenVersion and expiry.
- uid is the user id encoded through pkg/idcodec; mismatch between
  claim uid and session.UserID trips WAF block on the caller IP.
- identityHandler now rejects unknown/revoked/expired sessions, IP
  drift and stale TokenVersion. Refresh updates session.ExpiresAt.

User.TokenVersion bumps on password change and revokes outstanding
sessions, so a leaked JWT secret alone is no longer enough to forge
a token. JWTSession rows are GC'd every 10 minutes (expired + grace
or revoked >24h). OAuth2 callback shares the same issue path.

Includes regression tests for happy path, mismatched claim uid,
revoked session, TokenVersion bump, IP drift and unknown keyId.

Co-authored-by: cloudcode <cloudcode@users.noreply.github.com>

model/user.go

@@ -28,6 +28,7 @@ type User struct {
 	Role           Role   `json:"role,omitempty"`
 	AgentSecret    string `json:"agent_secret,omitempty" gorm:"type:char(32)"`
 	RejectPassword bool   `json:"reject_password,omitempty"`
+	TokenVersion   uint64 `json:"-" gorm:"not null;default:0"`
 }
 
 type UserInfo struct {