feat(backend): add OAuth2/JWT authentication support

- Add OAuth2 client for Authentik integration
- Implement JWT token generation and validation
- Add refresh token support with database storage
- Update database schema with oauth_subject, oauth_provider, and refresh_tokens table
- Create auth package with config, jwt, oauth, and token management
- Add OAuth endpoints: /auth/login, /auth/callback, /auth/refresh, /auth/logout
- Update AuthMiddleware to support both JWT and API key authentication
- Add user helper functions for OAuth user creation and retrieval
- Add .env.example with OAuth configuration template

API keys still work for CLI compatibility while JWT tokens support web/mobile clients.

opal-task/internal/auth/jwt.go

@@ -0,0 +1,57 @@
+package auth
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+type Claims struct {
+	UserID   int    `json:"user_id"`
+	Username string `json:"username"`
+	Email    string `json:"email,omitempty"`
+	jwt.RegisteredClaims
+}
+
+func GenerateJWT(userID int, username, email string, cfg *Config) (string, int64, error) {
+	expiresAt := time.Now().Add(time.Duration(cfg.JWTExpiry) * time.Second)
+
+	claims := &Claims{
+		UserID:   userID,
+		Username: username,
+		Email:    email,
+		RegisteredClaims: jwt.RegisteredClaims{
+			ExpiresAt: jwt.NewNumericDate(expiresAt),
+			IssuedAt:  jwt.NewNumericDate(time.Now()),
+			Issuer:    "opal-task",
+		},
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	signedToken, err := token.SignedString(cfg.JWTSecret)
+	if err != nil {
+		return "", 0, err
+	}
+
+	return signedToken, expiresAt.Unix(), nil
+}
+
+func ValidateJWT(tokenString string, cfg *Config) (*Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenString, &Claims{}, func(token *jwt.Token) (interface{}, error) {
+		if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
+		}
+		return cfg.JWTSecret, nil
+	})
+
+	if err != nil {
+		return nil, err
+	}
+
+	if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		return claims, nil
+	}
+
+	return nil, fmt.Errorf("invalid token")
+}