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/internal/db/db.go

@@ -0,0 +1,41 @@
+package db
+
+import (
+	"context"
+	_ "embed"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+//go:embed schema.sql
+var schema string
+
+func NewPool(ctx context.Context, dsn string) (*pgxpool.Pool, error) {
+	cfg, err := pgxpool.ParseConfig(dsn)
+	if err != nil {
+		return nil, fmt.Errorf("parse dsn: %w", err)
+	}
+	cfg.MaxConns = 25
+	cfg.MinConns = 2
+	cfg.MaxConnLifetime = 15 * time.Minute
+	cfg.MaxConnIdleTime = 5 * time.Minute
+
+	pool, err := pgxpool.NewWithConfig(ctx, cfg)
+	if err != nil {
+		return nil, fmt.Errorf("create pool: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		return nil, fmt.Errorf("ping db: %w", err)
+	}
+	return pool, nil
+}
+
+// InitSchema applies the embedded schema.sql (idempotent via IF NOT EXISTS).
+func InitSchema(ctx context.Context, pool *pgxpool.Pool) error {
+	if _, err := pool.Exec(ctx, schema); err != nil {
+		return fmt.Errorf("init schema: %w", err)
+	}
+	return nil
+}