feat(search+docs): trigram fuzzy search, brand/country filters, developer docs

Search:
- migration 0009: trigram GIN index on brand.name + btree on country_of_origin
- SearchProducts: typo-tolerant word_similarity matching (>=0.42) on top of
  ILIKE substring + barcode; new brand/country filters; rank by
  similarity * (0.5 + quality_score). Response gains country_of_origin,
  quality_score and per-result relevance score.
- public search UI: brand/country filter inputs; show country in results

Docs:
- serve embedded OpenAPI 3 spec at GET /api/v1/openapi.json (not rate limited)
- ApiDocs page: auth + rate-limit section, updated search params/response
- docs/api.md developer guide

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

public-frontend/src/api.ts

@@ -25,11 +25,22 @@ export interface SearchResult {
   total: number;
 }
 
+export interface SearchFilters {
+  brand?: string;
+  country?: string;
+}
+
 export const api = {
-  search: (q: string, page = 1, size = 20) =>
-    req<SearchResult>(
-      `/api/v1/products/search?q=${encodeURIComponent(q)}&page=${page}&size=${size}`,
-    ),
+  search: (q: string, page = 1, size = 20, filters: SearchFilters = {}) => {
+    const params = new URLSearchParams({
+      q,
+      page: String(page),
+      size: String(size),
+    });
+    if (filters.brand) params.set("brand", filters.brand);
+    if (filters.country) params.set("country", filters.country);
+    return req<SearchResult>(`/api/v1/products/search?${params.toString()}`);
+  },
   product: (id: string) => req<Product>(`/api/v1/products/${id}`),
   categories: () => req<{ items: Category[] }>(`/api/v1/categories`),
   submit: (input: SubmissionInput) =>