perf(llm): route background tasks to dedicated model to preserve KV cache

Background tasks (title generation, tag suggestions, project summaries,
RSS classification) were using qwen3:8b and wiping its KV cache after
every response, preventing prefix cache hits on subsequent user messages.

Adds OLLAMA_BACKGROUND_MODEL (default: qwen2.5:0.5b) config var and
routes all background LLM calls to it, keeping qwen3:8b's KV cache
warm between user messages for consistent sub-second TTFT.

Also adds infinite scroll to KnowledgeView (replaces load-more button)
and bakes spaCy en_core_web_sm into the Docker image to eliminate the
pip install on every startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/KnowledgeView.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed, watch, onMounted, onUnmounted, nextTick } from "vue";
+import { ref, computed, watch, onMounted, onUnmounted, nextTick, watchEffect } from "vue";
 import { useRouter } from "vue-router";
 import { apiGet, apiPatch, transcribeAudio } from "@/api/client";
 import { fmtCompact } from "@/utils/dateFormat";
@@ -320,17 +320,45 @@ function formatDate(iso: string): string {
   return d.toLocaleDateString(undefined, { month: "short", day: "numeric", year: "numeric" });
 }
 
+// ─── Infinite scroll ──────────────────────────────────────────────────────────
+
+const sentinelEl = ref<HTMLElement | null>(null);
+let scrollObserver: IntersectionObserver | null = null;
+
+function setupScrollObserver() {
+  if (scrollObserver) scrollObserver.disconnect();
+  if (!sentinelEl.value) return;
+  scrollObserver = new IntersectionObserver(
+    (entries) => {
+      if (entries[0].isIntersecting && !loading.value && items.value.length < total.value) {
+        page.value++;
+        fetchItems();
+      }
+    },
+    { rootMargin: "200px" }
+  );
+  scrollObserver.observe(sentinelEl.value);
+}
+
 // ─── Lifecycle ────────────────────────────────────────────────────────────────
 
 onMounted(() => {
   fetchItems(true);
   fetchTags();
   fetchTodayBar();
-  nextTick(() => chatInputEl.value?.focus());
+  nextTick(() => {
+    chatInputEl.value?.focus();
+    setupScrollObserver();
+  });
 });
 
 onUnmounted(() => {
   if (searchDebounce) clearTimeout(searchDebounce);
+  scrollObserver?.disconnect();
+});
+
+watchEffect(() => {
+  if (sentinelEl.value) setupScrollObserver();
 });
 </script>
 
@@ -499,11 +527,9 @@ onUnmounted(() => {
           </div>
         </div>
 
-        <!-- Load more -->
-        <div v-if="items.length < total && !loading" class="load-more">
-          <button class="btn-load-more" @click="() => { page++; fetchItems() }">
-            Load more ({{ total - items.length }} remaining)
-          </button>
+        <!-- Infinite scroll sentinel -->
+        <div ref="sentinelEl" class="scroll-sentinel">
+          <span v-if="loading && items.length > 0" class="sentinel-loading">Loading…</span>
         </div>
       </div>
 
@@ -975,23 +1001,18 @@ onUnmounted(() => {
 }
 .empty-hint { font-size: 0.85rem; opacity: 0.7; }
 
-/* ── Load more ───────────────────────────────────────────── */
-.load-more {
-  padding: 16px 20px;
-  text-align: center;
+/* ── Infinite scroll sentinel ────────────────────────────── */
+.scroll-sentinel {
+  height: 40px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
   flex-shrink: 0;
 }
-.btn-load-more {
-  padding: 8px 20px;
-  border-radius: 8px;
-  border: 1px solid var(--color-border, rgba(255,255,255,0.1));
-  background: transparent;
+.sentinel-loading {
+  font-size: 0.8rem;
   color: var(--color-muted);
-  cursor: pointer;
-  font-size: 0.85rem;
-  transition: all 0.15s;
 }
-.btn-load-more:hover { color: var(--color-text); border-color: rgba(255,255,255,0.2); }
 
 /* ── Graph panel ─────────────────────────────────────────── */
 .graph-panel {