security: add headers middleware, session regeneration, and body size limits

- Add SecurityHeaders middleware applied globally: HSTS, X-Content-Type-Options,
  X-Frame-Options, CSP, Referrer-Policy, and Permissions-Policy headers on every
  response.
- Add session regeneration (Regenerate method) after successful login to prevent
  session fixation attacks. Old session is destroyed and a new ID is issued.
- Add MaxBodySize middleware using http.MaxBytesReader to limit POST/PUT/PATCH
  request bodies to 1 MB on all form endpoints (/pages, /sources, /source/*).

Closes #34, closes #38, closes #39

internal/session/session.go

@@ -135,3 +135,50 @@ func (s *Session) Destroy(sess *sessions.Session) {
 	sess.Options.MaxAge = -1
 	s.ClearUser(sess)
 }
+
+// Regenerate creates a new session with the same values but a fresh ID.
+// The old session is destroyed (MaxAge = -1) and saved, then a new session
+// is created. This prevents session fixation attacks by ensuring the
+// session ID changes after privilege escalation (e.g. login).
+func (s *Session) Regenerate(r *http.Request, w http.ResponseWriter, oldSess *sessions.Session) (*sessions.Session, error) {
+	// Copy the values from the old session
+	oldValues := make(map[interface{}]interface{})
+	for k, v := range oldSess.Values {
+		oldValues[k] = v
+	}
+
+	// Destroy the old session
+	oldSess.Options.MaxAge = -1
+	s.ClearUser(oldSess)
+	if err := oldSess.Save(r, w); err != nil {
+		return nil, fmt.Errorf("failed to destroy old session: %w", err)
+	}
+
+	// Create a new session (gorilla/sessions generates a new ID)
+	newSess, err := s.store.New(r, SessionName)
+	if err != nil {
+		// store.New may return an error alongside a new empty session
+		// if the old cookie is now invalid. That is expected after we
+		// destroyed it above. Only fail on a nil session.
+		if newSess == nil {
+			return nil, fmt.Errorf("failed to create new session: %w", err)
+		}
+	}
+
+	// Restore the copied values into the new session
+	for k, v := range oldValues {
+		newSess.Values[k] = v
+	}
+
+	// Apply the standard session options (the destroyed old session had
+	// MaxAge = -1, which store.New might inherit from the cookie).
+	newSess.Options = &sessions.Options{
+		Path:     "/",
+		MaxAge:   86400 * 7,
+		HttpOnly: true,
+		Secure:   !s.config.IsDev(),
+		SameSite: http.SameSiteLaxMode,
+	}
+
+	return newSess, nil
+}