Report the latest failed suggestion batch on the deck

The deck reported only a pending or running batch, so a Discover page
opened after a batch failed (or was interrupted by a restart) had no job
to follow and showed "All caught up" or the remaining cards with no
error. GET /api/v1/suggestions/deck now also returns failed_batch: the
newest batch when it failed, read in the same snapshot after orphaned
batches are failed, so at most one of active_batch and failed_batch is
set and a later batch supersedes the failure. The page follows the
failed batch's job like the active one, so its failure shows with the
existing Retry, which asks for a new batch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/querier.go

@@ -78,6 +78,9 @@ type Querier interface {
 	// Returns sql.ErrNoRows until the row exists.
 	GetIntegrationsGoogle(ctx context.Context) (IntegrationsGoogle, error)
 	GetJob(ctx context.Context, id string) (Job, error)
+	// The newest batch, whatever its status. Returns sql.ErrNoRows when there
+	// is none.
+	GetLatestSuggestionBatch(ctx context.Context) (SuggestionBatch, error)
 	GetPhoto(ctx context.Context, id string) (Photo, error)
 	// Finds stored image bytes by content hash. The schema does not make sha256
 	// unique, so the earliest-created (lowest id) match is returned.