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>

CLAUDE.md

@@ -161,9 +161,9 @@ seeking through files, a flood of wrong passwords. Things about them:
 * The daemon is on **SQLite** unless `--postgres`, which puts it in `IPX_TEST_DATABASE_URL`, in
   a schema of its own (`ipx_load`) dropped and made again each run. That URL, in `/src/.envrc`, is
   `ipodderx_test` on the `databases` project's server, production's, as `ipodderx`, taken from
-  `ipx.env`; the harness refuses a database whose name does not end in `_test`. SQLite's numbers
-  carry its single connection (#136); Postgres's are what production would see, and a run loads
-  the server production's database is on, so not while someone is using iPX in earnest.
+  `ipx.env`; the harness refuses a database whose name does not end in `_test`. Postgres's
+  numbers are what production would see, and a run loads the server production's database is on,
+  so not while someone is using iPX in earnest. Each budget has a value for each database.
 * The Rust tests run on Postgres with the same URL: `. /src/.envrc && cargo test`. Each test
   makes a schema `ipxt_<pid>_<n>`, and the next run drops those an earlier one left.