fix: always set Secure/HttpOnly/SameSite on session cookies (closes #47)

Resolve the two remaining gosec G124 findings (internal/session/
session.go:84 and :128): session cookies are now unconditionally
Secure, HttpOnly, and SameSite=Strict on both the CreateSession
set-cookie path and the ClearSession delete-cookie path. gosec requires
these attributes to be constant, and there is no legitimate
configuration in which the authentication cookie should be weaker, so
the former secure toggle (wired to !config.Debug) is removed rather
than kept as a variable.

The toggle parameter on NewManager is retained as an ignored blank
parameter so existing call sites (including tests) keep compiling;
removing it is tracked as a Future Step in TODO.md. Local development
over http://localhost keeps working because browsers treat localhost as
a trustworthy origin and accept Secure cookies there.

Update TODO.md per its Workflow section: record this step as completed,
promote the manual auth/URL-flow test pass to Next Step, and correct
the stale Status text (make check is now green).

internal/session/session.go

@@ -36,14 +36,18 @@ type Data struct {
 
 // Manager handles session creation and validation using encrypted cookies.
 type Manager struct {
-	sc       *securecookie.SecureCookie
-	secure   bool // Set Secure flag on cookies (should be true in production)
-	sameSite http.SameSite
+	sc *securecookie.SecureCookie
 }
 
 // NewManager creates a session manager with keys derived from the signing key.
-// Set secure=true in production to require HTTPS for cookies.
-func NewManager(signingKey string, secure bool) (*Manager, error) {
+//
+// Session cookies always carry the Secure, HttpOnly, and SameSite=Strict
+// attributes; this cannot be configured. Browsers treat http://localhost as a
+// trustworthy origin and accept Secure cookies there, so local development
+// keeps working. The second parameter is the former secure toggle: it is
+// ignored and retained only so existing call sites keep compiling; it will be
+// removed in a follow-up change.
+func NewManager(signingKey string, _ bool) (*Manager, error) {
 	masterKey := []byte(signingKey)
 
 	// Derive separate keys for HMAC (hash) and encryption (block)
@@ -61,9 +65,7 @@ func NewManager(signingKey string, secure bool) (*Manager, error) {
 	sc.MaxAge(int(SessionTTL.Seconds()))
 
 	return &Manager{
-		sc:       sc,
-		secure:   secure,
-		sameSite: http.SameSiteStrictMode,
+		sc: sc,
 	}, nil
 }
 
@@ -87,8 +89,8 @@ func (m *Manager) CreateSession(w http.ResponseWriter) error {
 		Path:     "/",
 		MaxAge:   int(SessionTTL.Seconds()),
 		HttpOnly: true,
-		Secure:   m.secure,
-		SameSite: m.sameSite,
+		Secure:   true,
+		SameSite: http.SameSiteStrictMode,
 	})
 
 	return nil
@@ -131,8 +133,8 @@ func (m *Manager) ClearSession(w http.ResponseWriter) {
 		Path:     "/",
 		MaxAge:   -1, // Delete immediately
 		HttpOnly: true,
-		Secure:   m.secure,
-		SameSite: m.sameSite,
+		Secure:   true,
+		SameSite: http.SameSiteStrictMode,
 	})
 }