feat(backend): reject non-positive token TTLs at config load

Every duration in the config is a token TTL (access, refresh, content). A zero
or negative value mints already-expired tokens — no login, no media playback —
and previously loaded silently. parseDuration now rejects <= 0 with a clear
error, so misconfiguration fails fast at startup instead of mysteriously at
runtime. The AuthService itself stays permissive (it's constructed directly in
tests with arbitrary TTLs); config load is the gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/config/config_test.go

@@ -0,0 +1,57 @@
+package config
+
+import (
+	"strings"
+	"testing"
+)
+
+// setValidEnv sets every required variable to a valid dummy value, so a test can
+// then override one var to exercise a single validation path.
+func setValidEnv(t *testing.T) {
+	t.Helper()
+	t.Setenv("JWT_SECRET", "test-secret")
+	t.Setenv("ADMIN_PASSWORD", "test-password")
+	t.Setenv("DATABASE_URL", "postgres://u:p@localhost:5432/db?sslmode=disable")
+	t.Setenv("FILES_PATH", "/tmp/files")
+	t.Setenv("THUMBS_CACHE_PATH", "/tmp/thumbs")
+	t.Setenv("IMPORT_PATH", "/tmp/import")
+	// Pin the TTLs to valid values so an ambient env var can't perturb the case
+	// under test; individual tests override the one they exercise.
+	t.Setenv("JWT_ACCESS_TTL", "15m")
+	t.Setenv("JWT_REFRESH_TTL", "720h")
+	t.Setenv("CONTENT_TOKEN_TTL", "6h")
+}
+
+func TestLoadValid(t *testing.T) {
+	setValidEnv(t)
+	cfg, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if cfg.JWTAccessTTL <= 0 || cfg.JWTRefreshTTL <= 0 || cfg.ContentTokenTTL <= 0 {
+		t.Fatalf("TTLs should be positive: access=%v refresh=%v content=%v",
+			cfg.JWTAccessTTL, cfg.JWTRefreshTTL, cfg.ContentTokenTTL)
+	}
+}
+
+func TestLoadRejectsNonPositiveTTL(t *testing.T) {
+	cases := []struct{ key, val string }{
+		{"JWT_ACCESS_TTL", "0"},
+		{"JWT_REFRESH_TTL", "-1h"},
+		{"CONTENT_TOKEN_TTL", "0s"},
+	}
+	for _, tc := range cases {
+		t.Run(tc.key, func(t *testing.T) {
+			setValidEnv(t)
+			t.Setenv(tc.key, tc.val)
+
+			_, err := Load()
+			if err == nil {
+				t.Fatalf("expected error for %s=%q", tc.key, tc.val)
+			}
+			if !strings.Contains(err.Error(), tc.key) || !strings.Contains(err.Error(), "must be positive") {
+				t.Fatalf("error should name %s and mention positivity, got: %v", tc.key, err)
+			}
+		})
+	}
+}