Add initial implementation of gitea-mcp-forward-auth microservice

- Create build and release workflows for CI/CD
- Implement Dockerfile for multi-stage builds
- Add core authentication logic with token validation
- Include HTTP handler for authorization checks
- Set up Kubernetes deployment and service manifests
- Update README with usage instructions and configuration details

internal/auth/auth.go

@@ -0,0 +1,162 @@
+package auth
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+const EnvTokens = "AUTH_PROXY_TOKENS"
+
+var (
+	ErrMissingAuthHeader  = errors.New("missing authorization header")
+	ErrInvalidAuthHeader  = errors.New("invalid authorization header format")
+	ErrEmptyTokenSet      = errors.New("no tokens configured")
+	ErrTokenDirUnreadable = errors.New("unable to read token directory")
+)
+
+type Validator struct {
+	tokenDigests [][sha256.Size]byte
+}
+
+func NewValidator(tokens []string) (*Validator, error) {
+	if len(tokens) == 0 {
+		return nil, ErrEmptyTokenSet
+	}
+
+	unique := make(map[[sha256.Size]byte]struct{}, len(tokens))
+	digests := make([][sha256.Size]byte, 0, len(tokens))
+	for _, token := range tokens {
+		t := strings.TrimSpace(token)
+		if t == "" {
+			continue
+		}
+		digest := sha256.Sum256([]byte(t))
+		if _, exists := unique[digest]; exists {
+			continue
+		}
+		unique[digest] = struct{}{}
+		digests = append(digests, digest)
+	}
+
+	if len(digests) == 0 {
+		return nil, ErrEmptyTokenSet
+	}
+
+	return &Validator{tokenDigests: digests}, nil
+}
+
+func (v *Validator) IsValid(token string) bool {
+	token = strings.TrimSpace(token)
+	if token == "" || v == nil {
+		return false
+	}
+
+	provided := sha256.Sum256([]byte(token))
+	matched := 0
+	for _, allowed := range v.tokenDigests {
+		matched |= subtle.ConstantTimeCompare(provided[:], allowed[:])
+	}
+
+	return matched == 1
+}
+
+func (v *Validator) TokenCount() int {
+	if v == nil {
+		return 0
+	}
+	return len(v.tokenDigests)
+}
+
+func ParseBearerToken(authHeader string) (string, error) {
+	authHeader = strings.TrimSpace(authHeader)
+	if authHeader == "" {
+		return "", ErrMissingAuthHeader
+	}
+
+	parts := strings.Fields(authHeader)
+	if len(parts) != 2 || !strings.EqualFold(parts[0], "Bearer") || strings.TrimSpace(parts[1]) == "" {
+		return "", ErrInvalidAuthHeader
+	}
+
+	return parts[1], nil
+}
+
+func LoadTokens(tokensDir, tokensCSV string) ([]string, error) {
+	tokens := make([]string, 0)
+
+	dirTokens, err := readTokensFromDir(tokensDir)
+	if err != nil {
+		return nil, err
+	}
+	tokens = append(tokens, dirTokens...)
+	tokens = append(tokens, parseTokensCSV(tokensCSV)...)
+
+	cleaned := make([]string, 0, len(tokens))
+	for _, token := range tokens {
+		t := strings.TrimSpace(token)
+		if t != "" {
+			cleaned = append(cleaned, t)
+		}
+	}
+
+	if len(cleaned) == 0 {
+		return nil, ErrEmptyTokenSet
+	}
+
+	return cleaned, nil
+}
+
+func readTokensFromDir(tokensDir string) ([]string, error) {
+	if strings.TrimSpace(tokensDir) == "" {
+		return nil, nil
+	}
+
+	entries, err := os.ReadDir(tokensDir)
+	if err != nil {
+		return nil, fmt.Errorf("%w %q: %v", ErrTokenDirUnreadable, tokensDir, err)
+	}
+
+	tokens := make([]string, 0, len(entries))
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+		path := filepath.Join(tokensDir, entry.Name())
+		content, err := os.ReadFile(path)
+		if err != nil {
+			return nil, fmt.Errorf("read token file %q: %w", path, err)
+		}
+		tokens = append(tokens, strings.TrimSpace(string(content)))
+	}
+
+	return tokens, nil
+}
+
+func parseTokensCSV(tokensCSV string) []string {
+	if strings.TrimSpace(tokensCSV) == "" {
+		return nil
+	}
+	parts := strings.Split(tokensCSV, ",")
+	tokens := make([]string, 0, len(parts))
+	for _, part := range parts {
+		t := strings.TrimSpace(part)
+		if t != "" {
+			tokens = append(tokens, t)
+		}
+	}
+	return tokens
+}
+
+func TokenFingerprint(token string) string {
+	token = strings.TrimSpace(token)
+	if token == "" {
+		return ""
+	}
+	sum := sha256.Sum256([]byte(token))
+	return fmt.Sprintf("sha256:%x", sum[:6])
+}