Fix review issues: detached contexts, loop device leak, timer leak, size_bytes

- Use context.Background() with timeout in destroySandbox/failBuild so
  cleanup and DB writes survive parent context cancellation on shutdown
- Fix loop device refcount leak in FlattenRootfs when dmDevice is nil
- Replace time.After with time.NewTimer in healthcheck polling to avoid
  goroutine leak when healthcheck passes early
- Capture size_bytes from CreateSnapshot/FlattenRootfs RPC responses
  instead of hardcoding 0 in the templates table insert
- Avoid leaking internal error details to API clients in build handler

internal/api/handlers_builds.go

@@ -3,6 +3,7 @@ package api
 import (
 	"encoding/json"
 	"fmt"
+	"log/slog"
 	"net/http"
 	"time"
 
@@ -119,7 +120,8 @@ func (h *buildHandler) Create(w http.ResponseWriter, r *http.Request) {
 		MemoryMB:     req.MemoryMB,
 	})
 	if err != nil {
-		writeError(w, http.StatusInternalServerError, "build_error", err.Error())
+		slog.Error("failed to create build", "error", err)
+		writeError(w, http.StatusInternalServerError, "build_error", "failed to create build")
 		return
 	}