Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

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

backend/.gitignore

@@ -0,0 +1,29 @@
+# Binaries
+/server
+/migrate
+/createuser
+*.exe
+
+# Build output
+dist/
+bin/
+
+# Uploads
+uploads/
+
+# Environment / Secrets
+.env
+*.env
+
+# IDE
+.idea/
+.vscode/
+*.swp
+
+# OS
+.DS_Store
+
+# Go test cache / coverage
+*.test
+*.out
+coverage.html