Custom RTMP saved accounts, RTMP test server, composition pipeline

- Backend: POST /providers/accounts/custom-rtmp to save reusable RTMP servers
- Backend: Encrypt rtmpUrl/streamKey in existing token fields, decrypt on GET
- Backend: Skip token revocation on DELETE for CUSTOM_RTMP accounts
- Backend: Decrypt CUSTOM_RTMP credentials into destinations on plan create/update
- Android: Add rtmpUrl/streamKey to LinkedAccount entity + shared parcelable (Room v6)
- Android: Add Custom RTMP dialog in AccountsScreen, auto-fill in plan destination picker
- Android: Handle CUSTOM_RTMP accounts in CreatePlanViewModel.loadExistingPlan
- Add local RTMP test server (tools/rtmp-server.js) with auto-ffplay on publish
- Add composition pipeline native code

app/src/main/cpp/egl_context.cpp

@@ -1,6 +1,7 @@
 #include "egl_context.h"
 
 #include <android/log.h>
+#include <android/native_window.h>
 #include <unistd.h>
 
 #define TAG "LckEglContext"
@@ -201,9 +202,58 @@ bool EglContext::SwapBuffers() {
     return eglSwapBuffers(display, surface) == EGL_TRUE;
 }
 
+bool EglContext::CreatePreviewSurface(ANativeWindow* window) {
+    if (!window || display == EGL_NO_DISPLAY) return false;
+
+    DestroyPreviewSurface();
+
+    previewSurface = eglCreateWindowSurface(display, config, window, nullptr);
+    if (previewSurface == EGL_NO_SURFACE) {
+        LOGE("eglCreateWindowSurface (preview) failed: 0x%x", eglGetError());
+        return false;
+    }
+
+    previewWindow = window;
+    eglQuerySurface(display, previewSurface, EGL_WIDTH, &previewWidth);
+    eglQuerySurface(display, previewSurface, EGL_HEIGHT, &previewHeight);
+    LOGI("Preview surface created: %dx%d", previewWidth, previewHeight);
+    return true;
+}
+
+void EglContext::DestroyPreviewSurface() {
+    if (previewSurface != EGL_NO_SURFACE && display != EGL_NO_DISPLAY) {
+        // Make sure preview isn't current before destroying
+        eglMakeCurrent(display, surface, surface, context);
+        eglDestroySurface(display, previewSurface);
+        previewSurface = EGL_NO_SURFACE;
+        LOGI("Preview surface destroyed");
+    }
+    if (previewWindow) {
+        ANativeWindow_release(previewWindow);
+        previewWindow = nullptr;
+    }
+    previewWidth = 0;
+    previewHeight = 0;
+}
+
+bool EglContext::MakePreviewCurrent() {
+    if (previewSurface == EGL_NO_SURFACE) return false;
+    return eglMakeCurrent(display, previewSurface, previewSurface, context) == EGL_TRUE;
+}
+
+bool EglContext::MakeEncoderCurrent() {
+    return eglMakeCurrent(display, surface, surface, context) == EGL_TRUE;
+}
+
+bool EglContext::SwapPreviewBuffers() {
+    if (previewSurface == EGL_NO_SURFACE) return false;
+    return eglSwapBuffers(display, previewSurface) == EGL_TRUE;
+}
+
 void EglContext::Release() {
     if (display != EGL_NO_DISPLAY) {
         eglMakeCurrent(display, EGL_NO_SURFACE, EGL_NO_SURFACE, EGL_NO_CONTEXT);
+        DestroyPreviewSurface();
         if (surface != EGL_NO_SURFACE) {
             eglDestroySurface(display, surface);
             surface = EGL_NO_SURFACE;