feat(modules): public GET /api/modules discovery endpoint

Add HandleGetModules returning module state, metadata, and
public config (auth providers, Create URI scheme). No auth
required — clients call this pre-login.

Register at /api/modules before the auth middleware.

Ref #97

internal/api/handlers.go

@@ -173,6 +173,54 @@ func (s *Server) HandleReady(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// HandleGetModules returns the public module discovery response.
+// No authentication required — clients call this pre-login.
+func (s *Server) HandleGetModules(w http.ResponseWriter, r *http.Request) {
+	mods := make(map[string]any, 10)
+	for _, m := range s.modules.All() {
+		entry := map[string]any{
+			"enabled":  s.modules.IsEnabled(m.ID),
+			"required": m.Required,
+			"name":     m.Name,
+		}
+		if m.Version != "" {
+			entry["version"] = m.Version
+		}
+		if len(m.DependsOn) > 0 {
+			entry["depends_on"] = m.DependsOn
+		}
+
+		// Public config (non-secret) for specific modules.
+		switch m.ID {
+		case "auth":
+			if s.cfg != nil {
+				entry["config"] = map[string]any{
+					"local_enabled":   s.cfg.Auth.Local.Enabled,
+					"ldap_enabled":    s.cfg.Auth.LDAP.Enabled,
+					"oidc_enabled":    s.cfg.Auth.OIDC.Enabled,
+					"oidc_issuer_url": s.cfg.Auth.OIDC.IssuerURL,
+				}
+			}
+		case "freecad":
+			if s.cfg != nil {
+				entry["config"] = map[string]any{
+					"uri_scheme": s.cfg.FreeCAD.URIScheme,
+				}
+			}
+		}
+
+		mods[m.ID] = entry
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"modules": mods,
+		"server": map[string]any{
+			"version":   "0.2",
+			"read_only": s.serverState.IsReadOnly(),
+		},
+	})
+}
+
 // Schema handlers
 
 // SchemaResponse represents a schema in API responses.

internal/api/routes.go

@@ -58,6 +58,7 @@ func NewRouter(server *Server, logger zerolog.Logger) http.Handler {
 	r.Get("/auth/callback", server.HandleOIDCCallback)
 
 	// Public API endpoints (no auth required)
+	r.Get("/api/modules", server.HandleGetModules)
 	r.Get("/api/auth/config", server.HandleAuthConfig)
 
 	// API routes (require auth, no CSRF — token auth instead)