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/go.mod

@@ -0,0 +1,20 @@
+module github.com/jacek/pamietnik/backend
+
+go 1.25.7
+
+require (
+	github.com/go-chi/chi/v5 v5.2.5
+	github.com/golang-migrate/migrate/v4 v4.19.1
+	github.com/jackc/pgx/v5 v5.8.0
+	golang.org/x/crypto v0.48.0
+)
+
+require (
+	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	golang.org/x/sync v0.19.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
+	golang.org/x/text v0.34.0 // indirect
+)