Remove the Go reference server; keep only Rust

The Go implementation is preserved in the v1.0.0 tag. Its observable
behaviour stays the API reference: responses recorded from the Go server
live in tools/compatibility_expected.txt, and tools/compatibility.py now
compares the Rust server against them (--update re-records after an
intentional change). The Go SQLite schema is frozen in
tests/fixtures/go_schema_v1.sql for the existing-database test.

Move the web client from internal/httpapi/web to web/, drop the Go CI job
and Go version pin, and update README, AGENTS.md and docs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/auth/tokens.go

@@ -1,90 +0,0 @@
-package auth
-
-import (
-	"crypto/hmac"
-	"crypto/rand"
-	"crypto/sha256"
-	"encoding/base64"
-	"encoding/hex"
-	"encoding/json"
-	"errors"
-	"strings"
-	"time"
-)
-
-type Claims struct {
-	Subject string `json:"sub"`
-	Email   string `json:"email"`
-	Issuer  string `json:"iss"`
-	Issued  int64  `json:"iat"`
-	Expires int64  `json:"exp"`
-	ID      string `json:"jti"`
-}
-
-type TokenManager struct {
-	secret    []byte
-	accessTTL time.Duration
-}
-
-func NewTokenManager(secret string, accessTTL time.Duration) TokenManager {
-	return TokenManager{secret: []byte(secret), accessTTL: accessTTL}
-}
-
-func (manager TokenManager) AccessToken(userID, email string) (string, time.Time, error) {
-	now := time.Now().UTC()
-	expires := now.Add(manager.accessTTL)
-	id, err := RandomToken(16)
-	if err != nil {
-		return "", time.Time{}, err
-	}
-	header, _ := json.Marshal(map[string]string{"alg": "HS256", "typ": "JWT"})
-	claims, _ := json.Marshal(Claims{Subject: userID, Email: email, Issuer: "balance-server", Issued: now.Unix(), Expires: expires.Unix(), ID: id})
-	unsigned := encode(header) + "." + encode(claims)
-	signature := manager.sign(unsigned)
-	return unsigned + "." + signature, expires, nil
-}
-
-func (manager TokenManager) Verify(token string) (Claims, error) {
-	parts := strings.Split(token, ".")
-	if len(parts) != 3 {
-		return Claims{}, errors.New("invalid token")
-	}
-	unsigned := parts[0] + "." + parts[1]
-	expected := manager.sign(unsigned)
-	if !hmac.Equal([]byte(parts[2]), []byte(expected)) {
-		return Claims{}, errors.New("invalid token signature")
-	}
-	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
-	if err != nil {
-		return Claims{}, errors.New("invalid token payload")
-	}
-	var claims Claims
-	if err := json.Unmarshal(payload, &claims); err != nil {
-		return Claims{}, errors.New("invalid token claims")
-	}
-	if claims.Issuer != "balance-server" || claims.Subject == "" || time.Now().UTC().Unix() >= claims.Expires {
-		return Claims{}, errors.New("expired or invalid token")
-	}
-	return claims, nil
-}
-
-func RandomToken(bytes int) (string, error) {
-	value := make([]byte, bytes)
-	if _, err := rand.Read(value); err != nil {
-		return "", err
-	}
-	return base64.RawURLEncoding.EncodeToString(value), nil
-}
-
-func TokenHash(token string) string {
-	hash := sha256.Sum256([]byte(token))
-	return hex.EncodeToString(hash[:])
-}
-
-func (manager TokenManager) sign(value string) string {
-	mac := hmac.New(sha256.New, manager.secret)
-	_, _ = mac.Write([]byte(value))
-	return base64.RawURLEncoding.EncodeToString(mac.Sum(nil))
-}
-
-func encode(value []byte) string { return base64.RawURLEncoding.EncodeToString(value) }