fix(embed): chunk + mean-pool long text so large pages embed

Split long page text into 1500-char chunks before calling Ollama, then
mean-pool the per-chunk vectors into one page vector. Removes the hard
6000-char slice that still caused 500s on dense markdown/table pages.

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

tests/ai/embed_chunking.test.js

@@ -0,0 +1,36 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { chunkText, embedTextPooled } from '../../lib/ai/ollama.js';
+
+afterEach(() => { vi.unstubAllGlobals(); });
+
+describe('chunkText', () => {
+  it('returns [] for empty', () => { expect(chunkText('')).toEqual([]); });
+  it('keeps short text as one chunk', () => { expect(chunkText('hello\nworld', 1500)).toEqual(['hello\nworld']); });
+  it('splits long text into <=size chunks covering all chars', () => {
+    const text = Array.from({length: 50}, (_,i)=>`line ${i} ${'x'.repeat(40)}`).join('\n');
+    const chunks = chunkText(text, 200);
+    expect(chunks.length).toBeGreaterThan(1);
+    for (const c of chunks) expect(c.length).toBeLessThanOrEqual(200);
+  });
+  it('hard-splits a single over-long line', () => {
+    const chunks = chunkText('y'.repeat(500), 100);
+    expect(chunks.length).toBe(5);
+    expect(chunks.every(c => c.length <= 100)).toBe(true);
+  });
+});
+
+describe('embedTextPooled', () => {
+  it('mean-pools chunk vectors', async () => {
+    // two chunks (size 5 forces split), fetch returns embedding = [callCount, callCount]
+    let n = 0;
+    vi.stubGlobal('fetch', vi.fn(async () => { n++; return { ok: true, json: async () => ({ embedding: [n, n] }) }; }));
+    const v = await embedTextPooled('aaaaa\nbbbbb', { chunkSize: 5 });
+    // chunks: ['aaaaa','bbbbb'] -> vectors [1,1],[2,2] -> mean [1.5,1.5]
+    expect(v).toEqual([1.5, 1.5]);
+  });
+  it('single chunk equals single embed', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => ({ ok: true, json: async () => ({ embedding: [7, 8, 9] }) })));
+    const v = await embedTextPooled('short', { chunkSize: 1500 });
+    expect(v).toEqual([7, 8, 9]);
+  });
+});