Add UUID-based template IDs and team-scoped template directory layout

Introduces internal/layout package for centralized path construction,
migrates templates from name-based TEXT primary keys to UUID PKs with
team-scoped directories (WRENN_DIR/images/teams/{team_id}/{template_id}).
The built-in minimal template uses sentinel zero UUIDs. Proto messages
carry team_id + template_id alongside deprecated template name field.
Team deletion now cleans up template files across all hosts.

internal/api/middleware.go

@@ -12,6 +12,9 @@ import (
 	"time"
 
 	"connectrpc.com/connect"
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.omukk.dev/wrenn/sandbox/internal/id"
 )
 
 type errorResponse struct {
@@ -35,6 +38,11 @@ func writeError(w http.ResponseWriter, status int, code, message string) {
 	})
 }
 
+// formatUUIDForRPC converts a pgtype.UUID to a hex string for RPC messages.
+func formatUUIDForRPC(u pgtype.UUID) string {
+	return id.UUIDString(u)
+}
+
 // agentErrToHTTP maps a Connect RPC error to an HTTP status, error code, and message.
 func agentErrToHTTP(err error) (int, string, string) {
 	switch connect.CodeOf(err) {