fix(discovery): render empty queue as [] not null (500 on empty prod)

Go's nil slice marshals as JSON null, not [], which crashed the Svelte
page's .length access on fresh installs where no discovery tick has
happened yet. Reproduced in production: /admin/discovery → 500 because
data.queue was null and {queue.length} dereferenced it.

Backend: initialize every returning slice in repository.go via
make([]T, 0) so zero rows serialize as [] consistently. Also applies to
PickStaleBuckets, ListSeriesByCity, and Stats.RecentErrors.

Web: coalesce data.queue / data.stats.recent_errors at the top of the
Svelte script with `?? []` so future nil-slice regressions don't take
the whole page down.

backend/internal/domain/discovery/repository.go

@@ -57,7 +57,7 @@ LIMIT $2`
 		return nil, fmt.Errorf("pick buckets: %w", err)
 	}
 	defer rows.Close()
-	var out []Bucket
+	out := make([]Bucket, 0)
 	for rows.Next() {
 		var b Bucket
 		if err := rows.Scan(&b.ID, &b.Land, &b.Region, &b.YearMonth, &b.LastQueriedAt, &b.LastError, &b.CreatedAt); err != nil {
@@ -89,7 +89,7 @@ func (r *pgRepository) ListSeriesByCity(ctx context.Context, cityNormalized stri
 		return nil, err
 	}
 	defer rows.Close()
-	var out []SeriesCandidate
+	out := make([]SeriesCandidate, 0)
 	for rows.Next() {
 		var c SeriesCandidate
 		if err := rows.Scan(&c.ID, &c.Name, &c.City); err != nil {
@@ -155,7 +155,7 @@ LIMIT $2 OFFSET $3`, status, limit, offset)
 		return nil, err
 	}
 	defer rows.Close()
-	var out []DiscoveredMarket
+	out := make([]DiscoveredMarket, 0)
 	for rows.Next() {
 		var d DiscoveredMarket
 		if err := rows.Scan(
@@ -249,6 +249,7 @@ LIMIT $1`, recentErrorsLimit)
 		return Stats{}, fmt.Errorf("stats recent_errors: %w", err)
 	}
 	defer rows.Close()
+	s.RecentErrors = make([]BucketError, 0)
 	for rows.Next() {
 		var e BucketError
 		if err := rows.Scan(&e.Land, &e.Region, &e.YearMonth, &e.LastError, &e.LastQueriedAt); err != nil {

web/src/routes/admin/discovery/+page.svelte

@@ -2,6 +2,10 @@
 	import { enhance } from '$app/forms';
 	let { data } = $props();
 
+	// Coalesce nullable list fields (Go encodes nil slices as null).
+	const queue = $derived(data.queue ?? []);
+	const recentErrors = $derived(data.stats.recent_errors ?? []);
+
 	const lastTickLabel = $derived.by(() => {
 		if (!data.stats.last_tick_at) return 'nie';
 		const ts = new Date(data.stats.last_tick_at).getTime();
@@ -64,16 +68,16 @@
 		</div>
 	</div>
 
-	{#if data.stats.recent_errors && data.stats.recent_errors.length > 0}
+	{#if recentErrors.length > 0}
 		<details
 			class="mt-3 rounded border border-red-200 bg-red-50 p-2 text-sm dark:border-red-800 dark:bg-red-950"
 		>
 			<summary class="cursor-pointer text-red-700 dark:text-red-300">
-				{data.stats.recent_errors.length} bucket{data.stats.recent_errors.length === 1 ? '' : 's'}
+				{recentErrors.length} bucket{recentErrors.length === 1 ? '' : 's'}
 				with errors
 			</summary>
 			<ul class="mt-2 space-y-1 text-xs">
-				{#each data.stats.recent_errors as e}
+				{#each recentErrors as e}
 					<li class="font-mono">
 						<span class="text-stone-500">{e.last_queried_at?.slice(0, 16)}</span>
 						<span class="ml-2">{e.land} / {e.region} / {e.year_month}</span>
@@ -86,10 +90,10 @@
 
 	<h2 class="mt-8 text-lg font-semibold">Queue</h2>
 	<p class="mt-1 text-sm text-stone-500">
-		{data.queue.length} pending · showing from offset {data.offset}
+		{queue.length} pending · showing from offset {data.offset}
 	</p>
 
-	{#if data.queue.length === 0}
+	{#if queue.length === 0}
 		<p class="mt-8 rounded border border-stone-200 bg-stone-50 p-6 text-center text-stone-600">
 			Keine Einträge in der Warteschlange.
 		</p>
@@ -109,7 +113,7 @@
 				</tr>
 			</thead>
 			<tbody>
-				{#each data.queue as row (row.id)}
+				{#each queue as row (row.id)}
 					<tr class="border-b border-stone-100">
 						<td class="py-2">{row.land}</td>
 						<td>{row.bundesland}</td>