feat(search): persistenter Thumbnail-Cache in SQLite, Default-TTL 30 Tage

Vorher: In-Memory-Map, TTL 30 Minuten. Container-Neustart verwarf den
kompletten Cache, also musste nach jedem Deploy jede Suche wieder alle
Seiten laden.

Jetzt:
- Neue Tabelle thumbnail_cache (url PK, image, expires_at)
- Default-TTL 30 Tage, per Env KOCHWAS_THUMB_TTL_DAYS konfigurierbar
  (7, 365, was der User will — is alles ok laut Nutzer)
- Negative Cache: Seiten ohne Bild werden mit image=NULL gespeichert,
  damit wir nicht jede Suche die gleiche kaputte Seite wieder laden
- Lazy-Cleanup: pro searchWeb-Aufruf werden abgelaufene Zeilen via
  DELETE ... WHERE expires_at <= now() weggeräumt (Index-Scan, billig)

Migration 003_thumbnail_cache.sql: nicht-destruktiv, nur neue Tabelle.
Bestehende DB bekommt sie beim nächsten Start automatisch dazu.

Tests (99/99):
- Neuer Cache-Test: zweiter searchWeb für dieselbe URL macht keinen
  Page-Fetch mehr und liest die image-Spalte aus SQLite.

src/lib/server/db/migrations/003_thumbnail_cache.sql

@@ -0,0 +1,10 @@
+-- Long-term cache for page → image URL mappings extracted via og:image,
+-- JSON-LD, or first content <img>. Fetching every recipe page on every
+-- search is expensive; store the mapping with a 30-day default TTL.
+CREATE TABLE thumbnail_cache (
+  url        TEXT PRIMARY KEY,
+  image      TEXT,          -- NULL = page has no image (cache the negative too)
+  expires_at TEXT NOT NULL  -- ISO-8601 UTC
+);
+
+CREATE INDEX idx_thumbnail_cache_expires ON thumbnail_cache(expires_at);

src/lib/server/search/searxng.ts

@@ -179,14 +179,38 @@ function extractPageImage(html: string, baseUrl: string): string | null {
   }
 }
 
-type ThumbCacheEntry = { image: string | null; expires: number };
-const thumbCache = new Map<string, ThumbCacheEntry>();
-const THUMB_TTL_MS = 30 * 60 * 1000;
+const THUMB_TTL_DAYS = Number(process.env.KOCHWAS_THUMB_TTL_DAYS ?? 30);
+const THUMB_TTL_MS = THUMB_TTL_DAYS * 24 * 60 * 60 * 1000;
 
-async function enrichThumbnail(url: string): Promise<string | null> {
-  const now = Date.now();
-  const cached = thumbCache.get(url);
-  if (cached && cached.expires > now) return cached.image;
+function readCachedThumbnail(
+  db: Database.Database,
+  url: string
+): { image: string | null } | null {
+  const row = db
+    .prepare<[string, string], { image: string | null }>(
+      "SELECT image FROM thumbnail_cache WHERE url = ? AND expires_at > ?"
+    )
+    .get(url, new Date().toISOString());
+  return row ?? null;
+}
+
+function writeCachedThumbnail(
+  db: Database.Database,
+  url: string,
+  image: string | null
+): void {
+  const expiresAt = new Date(Date.now() + THUMB_TTL_MS).toISOString();
+  db.prepare(
+    'INSERT OR REPLACE INTO thumbnail_cache (url, image, expires_at) VALUES (?, ?, ?)'
+  ).run(url, image, expiresAt);
+}
+
+async function enrichThumbnail(
+  db: Database.Database,
+  url: string
+): Promise<string | null> {
+  const cached = readCachedThumbnail(db, url);
+  if (cached) return cached.image;
   let image: string | null = null;
   try {
     const html = await fetchText(url, { timeoutMs: 4_000, maxBytes: 512 * 1024 });
@@ -194,23 +218,30 @@ async function enrichThumbnail(url: string): Promise<string | null> {
   } catch {
     image = null;
   }
-  thumbCache.set(url, { image, expires: now + THUMB_TTL_MS });
+  writeCachedThumbnail(db, url, image);
   return image;
 }
 
-async function enrichAllThumbnails(hits: WebHit[]): Promise<void> {
+async function enrichAllThumbnails(
+  db: Database.Database,
+  hits: WebHit[]
+): Promise<void> {
   // Always fetch the page image even when SearXNG gave us a thumbnail —
   // the search engine's thumbnail is typically 150-200px, while og:image
   // / JSON-LD image on the page is the full-resolution recipe photo.
-  // The 30-min URL cache makes repeat searches instant.
+  // The thumbnail_cache table (default 30-day TTL) makes repeat searches instant.
   if (hits.length === 0) return;
+  // Lazy cleanup of expired entries — O(log n) index scan, cheap.
+  db.prepare('DELETE FROM thumbnail_cache WHERE expires_at <= ?').run(
+    new Date().toISOString()
+  );
   const queue = [...hits];
   const LIMIT = 6;
   const workers = Array.from({ length: Math.min(LIMIT, queue.length) }, async () => {
     while (queue.length > 0) {
       const h = queue.shift();
       if (!h) break;
-      const image = await enrichThumbnail(h.url);
+      const image = await enrichThumbnail(db, h.url);
       if (image) h.thumbnail = image;
     }
   });
@@ -272,7 +303,7 @@ export async function searchWeb(
     if (hits.length >= limit) break;
   }
   if (opts.enrichThumbnails !== false) {
-    await enrichAllThumbnails(hits);
+    await enrichAllThumbnails(db, hits);
   }
   return hits;
 }