feat: migrate passwords from SHA-256 to bcrypt

- Replace SHA-256 hex hashing with bcrypt (cost 10) for password storage
- VerifyPassword now uses bcrypt.CompareHashAndPassword
- HashPassword returns (string, error) instead of string
- Add IsBcryptHash helper to detect legacy hashes for future migration
- Remove duplicate verifyPassword from api.go (already done in prev commit)
- Promote golang.org/x/crypto to direct dependency

internal/auth/auth.go

@@ -4,16 +4,17 @@ package auth
 import (
 	"context"
 	"crypto/rand"
-	"crypto/sha256"
-	"crypto/subtle"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
 	"strings"
 	"time"
 
+	"golang.org/x/crypto/bcrypt"
+
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/config"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/database"
 )
@@ -380,10 +381,10 @@ func ExtractBearer(h string) string {
 	return ""
 }
 
-// VerifyPassword checks a plaintext password against a SHA-256 hex hash.
+// VerifyPassword checks a plaintext password against a stored bcrypt hash.
 func VerifyPassword(password, hash string) bool {
-	h := sha256.Sum256([]byte(password))
-	return subtle.ConstantTimeCompare([]byte(hex.EncodeToString(h[:])), []byte(hash)) == 1
+	err := bcrypt.CompareHashAndPassword([]byte(hash), []byte(password))
+	return err == nil
 }
 
 // GenerateToken creates a random hex token (16 bytes → 32 hex chars).
@@ -406,12 +407,24 @@ func writeError(w http.ResponseWriter, status int, err, msg string) {
 	})
 }
 
-// HashPassword returns the SHA-256 hex of a password for storage.
-func HashPassword(password string) string {
-	h := sha256.Sum256([]byte(password))
-	return hex.EncodeToString(h[:])
+// HashPassword returns a bcrypt hash of the password for storage.
+func HashPassword(password string) (string, error) {
+	hash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return "", fmt.Errorf("hashing password: %w", err)
+	}
+	return string(hash), nil
 }
 
+// IsBcryptHash reports whether the given hash looks like a bcrypt hash
+// (starts with $2a$, $2b$, or $2y$). Used to detect legacy SHA-256 hashes.
+func IsBcryptHash(hash string) bool {
+	return strings.HasPrefix(hash, "$2a$") || strings.HasPrefix(hash, "$2b$") || strings.HasPrefix(hash, "$2y$")
+}
+
+// ErrPasswordHashing is returned when bcrypt hashing fails.
+var ErrPasswordHashing = errors.New("password hashing failed")
+
 // GenerateUUID creates a random UUID v4-like string.
 func GenerateUUID() string {
 	b := make([]byte, 16)