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/api/routes/dashboard.py

@@ -83,6 +83,19 @@ async def dashboard_home(request: Request, db: Session = Depends(get_db)):
     })
 
 
+@router.get("/compose", response_class=HTMLResponse)
+async def dashboard_compose(request: Request, db: Session = Depends(get_db)):
+    """Página para crear nuevas publicaciones."""
+    user = require_auth(request, db)
+    if not user:
+        return RedirectResponse(url="/login", status_code=302)
+
+    return templates.TemplateResponse("compose.html", {
+        "request": request,
+        "user": user.to_dict()
+    })
+
+
 @router.get("/posts", response_class=HTMLResponse)
 async def dashboard_posts(request: Request, db: Session = Depends(get_db)):
     """Página de gestión de posts."""