feat(phase-2): Complete social media API integration

- Add ImageUploadService for public URL generation (Meta APIs)
- Create PublisherManager for unified multi-platform publishing
- Add /api/publish endpoints (single, multiple, thread, test)
- Add compose page in dashboard for creating posts
- Add connection test script (scripts/test_connections.py)
- Update navigation with compose link and logout

New endpoints:
- POST /api/publish/single - Publish to one platform
- POST /api/publish/multiple - Publish to multiple platforms
- POST /api/publish/thread - Publish thread (X/Threads)
- GET /api/publish/test - Test all API connections
- GET /api/publish/platforms - List available platforms

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/publishers/__init__.py

@@ -7,6 +7,7 @@ from app.publishers.x_publisher import XPublisher
 from app.publishers.threads_publisher import ThreadsPublisher
 from app.publishers.instagram_publisher import InstagramPublisher
 from app.publishers.facebook_publisher import FacebookPublisher
+from app.publishers.manager import PublisherManager, Platform, publisher_manager
 
 
 def get_publisher(platform: str) -> BasePublisher:
@@ -31,5 +32,8 @@ __all__ = [
     "ThreadsPublisher",
     "InstagramPublisher",
     "FacebookPublisher",
+    "PublisherManager",
+    "Platform",
+    "publisher_manager",
     "get_publisher"
 ]