Add authentication, authorization, and team-scoped access control

Implement email/password auth with JWT sessions and API key auth for
sandbox lifecycle. Users get a default team on signup; sandboxes,
snapshots, and API keys are scoped to teams.

- Add user, team, users_teams, and team_api_keys tables (goose migrations)
- Add JWT middleware (Bearer token) for user management endpoints
- Add API key middleware (X-API-Key header, SHA-256 hashed) for sandbox ops
- Add signup/login handlers with transactional user+team creation
- Add API key CRUD endpoints (create/list/delete)
- Replace owner_id with team_id on sandboxes and templates
- Update all handlers to use team-scoped queries
- Add godotenv for .env file loading
- Update OpenAPI spec and test UI with auth flows

internal/auth/jwt.go

@@ -0,0 +1,51 @@
+package auth
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+const jwtExpiry = 6 * time.Hour
+
+// Claims are the JWT payload.
+type Claims struct {
+	TeamID string `json:"team_id"`
+	Email  string `json:"email"`
+	jwt.RegisteredClaims
+}
+
+// SignJWT signs a new 6-hour JWT for the given user.
+func SignJWT(secret []byte, userID, teamID, email string) (string, error) {
+	now := time.Now()
+	claims := Claims{
+		TeamID: teamID,
+		Email:  email,
+		RegisteredClaims: jwt.RegisteredClaims{
+			Subject:   userID,
+			IssuedAt:  jwt.NewNumericDate(now),
+			ExpiresAt: jwt.NewNumericDate(now.Add(jwtExpiry)),
+		},
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(secret)
+}
+
+// VerifyJWT parses and validates a JWT, returning the claims on success.
+func VerifyJWT(secret []byte, tokenStr string) (Claims, error) {
+	token, err := jwt.ParseWithClaims(tokenStr, &Claims{}, func(t *jwt.Token) (any, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return secret, nil
+	})
+	if err != nil {
+		return Claims{}, fmt.Errorf("invalid token: %w", err)
+	}
+	c, ok := token.Claims.(*Claims)
+	if !ok || !token.Valid {
+		return Claims{}, fmt.Errorf("invalid token claims")
+	}
+	return *c, nil
+}