fix(search): support PLZ filter on home page

The home page dropped `plz` server-side, so /markets was called with
radius but no center (unfiltered) and the PLZ input rendered empty
after reload. +page.server.ts now reads plz, geocodes via /geocode,
and echoes plz back in searchParams for form rehydration.

Relaxes /geocode DTO + guard to accept PLZ without city — Nominatim
already supports postal-only lookups. URL lat/lon (GPS flow) take
priority over plz on tie-break; geocode failures fall through to no
geo-filter so the page always renders.

backend/internal/domain/market/dto.go

@@ -395,7 +395,7 @@ type SubmitMarketRequest struct {
 
 type GeocodeRequest struct {
 	Street  string `json:"street"`
-	City    string `json:"city" validate:"required"`
+	City    string `json:"city"`
 	Zip     string `json:"zip"`
 	Country string `json:"country"`
 }

backend/internal/pkg/geocode/nominatim.go

@@ -28,8 +28,8 @@ type nominatimResult struct {
 }
 
 func (g *Geocoder) Geocode(ctx context.Context, street, city, zip, country string) (*float64, *float64, error) {
-	if city == "" {
-		return nil, nil, fmt.Errorf("city is required for geocoding")
+	if city == "" && zip == "" {
+		return nil, nil, fmt.Errorf("city or zip is required for geocoding")
 	}
 
 	// Respect Nominatim rate limit: max 1 req/sec

web/src/routes/+page.server.ts

@@ -2,10 +2,34 @@ import type { PageServerLoad } from './$types.js';
 import { apiFetch, buildSearchQuery } from '$lib/api/client.js';
 import type { MarketSummary, PaginationMeta } from '$lib/api/types.js';
 
-export const load: PageServerLoad = async ({ url, fetch }) => {
-	const params: Record<string, string> = {};
+type Coords = { lat?: string; lon?: string };
 
+async function resolveCoords(
+	plz: string | null,
+	urlLat: string | null,
+	urlLon: string | null,
+	fetch: typeof globalThis.fetch
+): Promise<Coords> {
+	if (urlLat && urlLon) return { lat: urlLat, lon: urlLon };
+	if (!plz) return {};
+
+	try {
+		const res = await apiFetch<{ latitude: number | null; longitude: number | null }>('/geocode', {
+			method: 'POST',
+			body: JSON.stringify({ city: '', zip: plz, country: 'DE' }),
+			fetch
+		});
+		const { latitude, longitude } = res.data;
+		if (latitude == null || longitude == null) return {};
+		return { lat: String(latitude), lon: String(longitude) };
+	} catch {
+		return {};
+	}
+}
+
+export const load: PageServerLoad = async ({ url, fetch }) => {
 	const q = url.searchParams.get('q');
+	const plz = url.searchParams.get('plz');
 	const lat = url.searchParams.get('lat');
 	const lon = url.searchParams.get('lon');
 	const radius = url.searchParams.get('radius');
@@ -14,9 +38,12 @@ export const load: PageServerLoad = async ({ url, fetch }) => {
 	const sort = url.searchParams.get('sort');
 	const page = url.searchParams.get('page');
 
+	const coords = await resolveCoords(plz, lat, lon, fetch);
+
+	const params: Record<string, string> = {};
 	if (q) params.q = q;
-	if (lat) params.lat = lat;
-	if (lon) params.lon = lon;
+	if (coords.lat) params.lat = coords.lat;
+	if (coords.lon) params.lon = coords.lon;
 	if (radius) params.radius = radius;
 	if (from) params.from = from;
 	if (to) params.to = to;
@@ -26,34 +53,29 @@ export const load: PageServerLoad = async ({ url, fetch }) => {
 	const query = buildSearchQuery(params);
 	const path = `/markets${query ? `?${query}` : ''}`;
 
+	const searchParams = {
+		q: q ?? '',
+		plz: plz ?? '',
+		lat: lat ? Number(lat) : undefined,
+		lon: lon ? Number(lon) : undefined,
+		radius: radius ? Number(radius) : 25,
+		from: from ?? '',
+		to: to ?? '',
+		sort: sort ?? ''
+	};
+
 	try {
 		const res = await apiFetch<MarketSummary[]>(path, { fetch });
 		return {
 			markets: res.data,
 			meta: res.meta as PaginationMeta,
-			searchParams: {
-				q: q ?? '',
-				lat: lat ? Number(lat) : undefined,
-				lon: lon ? Number(lon) : undefined,
-				radius: radius ? Number(radius) : 25,
-				from: from ?? '',
-				to: to ?? '',
-				sort: sort ?? ''
-			}
+			searchParams
 		};
 	} catch {
 		return {
 			markets: [] as MarketSummary[],
 			meta: { page: 1, per_page: 20, total: 0, total_pages: 0 } as PaginationMeta,
-			searchParams: {
-				q: q ?? '',
-				lat: lat ? Number(lat) : undefined,
-				lon: lon ? Number(lon) : undefined,
-				radius: radius ? Number(radius) : 25,
-				from: from ?? '',
-				to: to ?? '',
-				sort: sort ?? ''
-			}
+			searchParams
 		};
 	}
 };

web/src/routes/+page.svelte

@@ -50,6 +50,7 @@
 
 	<SearchForm
 		q={data.searchParams.q}
+		plz={data.searchParams.plz}
 		radius={data.searchParams.radius}
 		from={data.searchParams.from}
 		to={data.searchParams.to}