Fix Railway backend deploy: Procfile + nixpacks.toml

Procfile: explicit start command as Railway fallback
nixpacks.toml: pin python311, install requirements.txt, set start command
These ensure Railway builds correctly when root directory is set to /backend

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Tawheed30

backend/Procfile

@@ -0,0 +1 @@
+web: uvicorn app.main:app --host 0.0.0.0 --port $PORT

backend/nixpacks.toml

@@ -0,0 +1,8 @@
+[phases.setup]
+nixPkgs = ["python311"]
+
+[phases.install]
+cmds = ["pip install -r requirements.txt"]
+
+[start]
+cmd = "uvicorn app.main:app --host 0.0.0.0 --port $PORT"