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/tokens.go

@@ -0,0 +1,91 @@
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+	"time"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+)
+
+func GenerateRefreshToken() (string, error) {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		return "", err
+	}
+	return base64.URLEncoding.EncodeToString(b), nil
+}
+
+func HashToken(token string) string {
+	hash := sha256.Sum256([]byte(token))
+	return base64.URLEncoding.EncodeToString(hash[:])
+}
+
+func StoreRefreshToken(userID int, token string, expiresIn int) error {
+	db := engine.GetDB()
+	if db == nil {
+		return fmt.Errorf("database not initialized")
+	}
+
+	hash := HashToken(token)
+	expiresAt := time.Now().Add(time.Duration(expiresIn) * time.Second).Unix()
+
+	_, err := db.Exec(`
+		INSERT INTO refresh_tokens (user_id, token_hash, expires_at, created_at)
+		VALUES (?, ?, ?, ?)
+	`, userID, hash, expiresAt, time.Now().Unix())
+
+	return err
+}
+
+func ValidateRefreshToken(token string) (int, error) {
+	db := engine.GetDB()
+	if db == nil {
+		return 0, fmt.Errorf("database not initialized")
+	}
+
+	hash := HashToken(token)
+
+	var userID int
+	var expiresAt int64
+	var revoked int
+
+	err := db.QueryRow(`
+		SELECT user_id, expires_at, revoked
+		FROM refresh_tokens
+		WHERE token_hash = ?
+	`, hash).Scan(&userID, &expiresAt, &revoked)
+
+	if err != nil {
+		return 0, err
+	}
+
+	if revoked == 1 {
+		return 0, fmt.Errorf("token revoked")
+	}
+
+	if time.Now().Unix() > expiresAt {
+		return 0, fmt.Errorf("token expired")
+	}
+
+	return userID, nil
+}
+
+func RevokeRefreshToken(token string) error {
+	db := engine.GetDB()
+	if db == nil {
+		return fmt.Errorf("database not initialized")
+	}
+
+	hash := HashToken(token)
+
+	_, err := db.Exec(`
+		UPDATE refresh_tokens
+		SET revoked = 1
+		WHERE token_hash = ?
+	`, hash)
+
+	return err
+}