fix(search): cap the encrypted-search IDB cache per room (bound disk growth)

The on-device search index grew unbounded over a long session. putRows now
prunes each touched room to MAX_ROWS_PER_ROOM (5000) — deleting the oldest rows
by [roomId, ts] via a self-chaining IDB cursor within the same write tx (never
awaits a non-IDB promise mid-tx, so the transaction can't auto-commit and
truncate the prune). Exposed a pure, unit-tested evictCount() for the decision;
the cursor path itself is browser-only (node --test has no IndexedDB).

Deliberate tradeoff (documented in code): the coverage window keeps claiming the
evicted tail so the search doesn't re-fetch → re-evict it forever. Net effect —
in a room past 5000 cached rows, an evicted old message is silently unsearchable
rather than churning. Clear cached index / logout still wipe everything.

Two review agents verified the IndexedDB-spec correctness (cursor delete+continue
semantics, put-then-count ordering, roomRange bracketing with no prefix bleed,
tx liveness, abort→cache-miss) since CI can't. Gate-green (tsc, eslint, prettier,
925 tests, build).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/searchCache.test.ts

@@ -2,6 +2,7 @@ import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import {
   computeCoverage,
+  evictCount,
   mergeSearchResults,
   putRows,
   queryRoom,
@@ -57,6 +58,18 @@ test('mergeSearchResults: missing ts sorts as 0 (last)', () => {
 
 const row = (ts: number): Pick<SearchCacheRow, 'ts'> => ({ ts });
 
+test('evictCount: 0 when under or at the cap, else the excess', () => {
+  assert.equal(evictCount(0, 100), 0);
+  assert.equal(evictCount(100, 100), 0); // exactly at cap → nothing evicted
+  assert.equal(evictCount(101, 100), 1);
+  assert.equal(evictCount(250, 100), 150);
+});
+
+test('evictCount: uses the default per-room cap (5000)', () => {
+  assert.equal(evictCount(5000), 0);
+  assert.equal(evictCount(5001), 1);
+});
+
 test('computeCoverage: derives oldest/newest from rows', () => {
   const cov = computeCoverage('!r', [row(30), row(10), row(20)], 3);
   assert.deepEqual(cov, { roomId: '!r', oldestTs: 10, newestTs: 30, count: 3 });