Give SQLite several connections, so requests are answered at once (#136)

SeaORM gives a SQLite pool one connection unless told otherwise, and connect() did not say, so
every request, every scan write and every read went through it in turn. Twenty-five people
browsing in the load tests (#133) got 50 requests a second out of 12 cores, the fastest answers
in 3-30ms and the medians ten times that.

SQLite now gets eight. In WAL readers run beside the one writer. Every transaction here writes
first, so it takes the write lock or waits out the busy timeout for it; SQLite refuses at once
only a transaction holding a snapshot a write has moved past, and none here does. Db::memory
switches its file to WAL too, as Db::open does, so the tests share it as the daemon does. A test
holds a write open and reads beside it, and fails with one connection.

The same browsing: a feed's items at p95 10ms where it was 133, a search 22 where 119, the feed
list 89 where 242. The load tests' SQLite budgets are tightened to match; the Rust tests take
2.8s where they took 7.5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - A feed whose address answers with nothing, as a lapsed domain does behind a DNS filter's block page, shows as failing, with what to do about it, instead of as a show that has not posted yet.
 - The search box finds episodes in Currently Listening. It did nothing there.
 - `ipx add`, `ipx rm` and `ipx import`, run while the daemon runs, reach it at once. It kept its old list of feeds and wrote it back at its next change, undoing them.
+- On SQLite, several requests are answered at once instead of one at a time. With 25 people browsing, a page of a feed's items comes back in 10ms rather than 130.
 - On Postgres, a write no longer waits for the database server's disk. A scan read about one feed a second and now reads 1,500 in 20 seconds.
 - Checking every feed no longer makes every icon in the feed list flash while it runs: the list keeps the icons it has already drawn.