perf: index stream_memories.created_at for the hero windows (#688)

ddiff --git a/migrations/0035_stream_memories_created_index.sql b/migrations/0035_stream_memories_created_index.sql
nnew file mode 100644
iindex 00000000..76c66d90
---- /dev/null
++++ b/migrations/0035_stream_memories_created_index.sql
@@@ -0,0 +1,6 @@
++-- phase: expand
++-- no-transaction
++-- The home hero's day/week/month/previous-week memory counts (server/db/database.js winCount) filter
++-- stream_memories by created_at 4 times per stats read: 3,306 + 3,306 + ... calls at 68 ms mean on 2026-11-08,
++-- each a 22 MB sequential scan. created_at has no index; a plain btree serves the >= and BETWEEN ranges.
++CREATE INDEX CONCURRENTLY IF NOT EXISTS stream_memories_created_at ON stream_memories (created_at);
ddiff --git a/test/stream-memories-created-index.test.js b/test/stream-memories-created-index.test.js
nnew file mode 100644
iindex 00000000..0969efab
---- /dev/null
++++ b/test/stream-memories-created-index.test.js
@@@ -0,0 +1,30 @@
++'use strict';
++/**
++ * The home hero counts stream_memories in four rolling windows by created_at (server/db/database.js winCount).
++ * On the 2026-11-08 production read each of those was a 68 ms sequential scan of the 22 MB table. Migration 0035
++ * adds a plain btree; this proves the range filter can use it and still counts the same rows.
++ */
++const assert = require('assert');
++
++(async () => {
++    const db = globalThis.__ovLiveTestDb;
++    assert.ok(db, 'the test preload provides the migrated database');
++
++    const indexes = await db.prepare(`SELECT indexname FROM pg_indexes
++        WHERE schemaname = current_schema() AND tablename = 'stream_memories'`).all();
++    assert.ok(indexes.some((r) => r.indexname === 'stream_memories_created_at'), 'migration 0035 created the index');
++
++    await db.prepare(`INSERT INTO users (id, username, password_hash, created_at) OVERRIDING SYSTEM VALUE VALUES (900000, 'index-test', 'x', datetime('now', '-2 days'))`).run();
++    await db.prepare(`INSERT INTO streams (id, user_id, title, is_live, created_at) OVERRIDING SYSTEM VALUE VALUES (900001, 900000, 'index test', 0, datetime('now', '-2 days'))`).run();
++    await db.prepare(`INSERT INTO stream_memories (stream_id, offset_seconds, description, created_at) VALUES
++        (900001, 1, 'recent', datetime('now', '-1 hour')), (900001, 2, 'old', datetime('now', '-40 days'))`).run();
++
++    const n = await db.prepare(`SELECT COUNT(*) AS c FROM stream_memories WHERE created_at >= datetime('now', '-1 day')`).get();
++    assert.strictEqual(Number(n.c), 1, 'the day window still counts only the recent memory');
++
++    await db.query('SET enable_seqscan = off');
++    const plan = (await db.prepare(`EXPLAIN SELECT COUNT(*) FROM stream_memories WHERE created_at >= datetime('now', '-1 day')`).all())
++        .map((r) => r['QUERY PLAN']).join('\n');
++    assert.match(plan, /stream_memories_created_at/, plan);
++    console.log('ok - the memory window count is an index scan over stream_memories_created_at');
++})().catch((err) => { console.error(err); process.exit(1); });