feat: jwt_generate, jwt_validate, password_hash, password_verify

Fase 2 del issue 0010 — auth core:
- jwt_generate/validate: HS256 manual con crypto/hmac + crypto/sha256
- password_hash/verify: wrappers de golang.org/x/crypto/bcrypt (cost 12 default)
- JWT rechaza alg != HS256 para mitigar ataque 'alg=none'
- hmac.Equal para comparacion constant-time de firmas

functions/infra/jwt_generate.go

@@ -0,0 +1,44 @@
+package infra
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+// JWTGenerate codifica un JWT firmado con HMAC-SHA256 (alg: HS256).
+// Si claims.IssuedAt viene en cero se setea a time.Now().Unix().
+// Retorna el token en formato "header.payload.signature" con los tres segmentos
+// codificados en base64url sin padding.
+func JWTGenerate(claims JWTClaims, secret string) (string, error) {
+	if secret == "" {
+		return "", errors.New("jwt_generate: secret vacio")
+	}
+	if claims.IssuedAt == 0 {
+		claims.IssuedAt = time.Now().Unix()
+	}
+
+	header := map[string]string{"alg": "HS256", "typ": "JWT"}
+	headerJSON, err := json.Marshal(header)
+	if err != nil {
+		return "", err
+	}
+	payloadJSON, err := json.Marshal(claims)
+	if err != nil {
+		return "", err
+	}
+
+	enc := base64.RawURLEncoding
+	headerPart := enc.EncodeToString(headerJSON)
+	payloadPart := enc.EncodeToString(payloadJSON)
+	signingInput := headerPart + "." + payloadPart
+
+	mac := hmac.New(sha256.New, []byte(secret))
+	mac.Write([]byte(signingInput))
+	sigPart := enc.EncodeToString(mac.Sum(nil))
+
+	return signingInput + "." + sigPart, nil
+}