SeaORM beside rusqlite: entities, schema sync, a second connection

The first step of moving to SeaORM (#18, phase 1). Nothing a user sees changes.

- src/entity.rs: the seven tables as SeaORM entities, matching the SQLite schema.
  Strings are Text, as the columns are; yes/no columns are bool, which is BOOLEAN
  on Postgres and stays INTEGER in the existing SQLite file (sync notes the
  difference and leaves it alone).
- Db holds a SeaORM connection to the same SQLite file beside the rusqlite one;
  functions move to it one at a time, and rusqlite goes with the last of them.
- db::sync creates what a database is missing from the entities (SeaORM's
  schema-sync, experimental, so sea-orm is pinned to ~2.0), plus the two indexes
  an entity cannot express. Checked against a copy of production: it added the
  lower(name) index and changed nothing else.
- Test databases are now built from the entities alone, in a temporary file
  (two connections to one ":memory:" are two databases), so every test also
  checks that the entities describe what the queries need. That caught the one
  difference: finding a user by name relied on COLLATE NOCASE, which Postgres
  lacks; it now compares lower() on both sides.
- rusqlite steps back to 0.39: 0.40's libsqlite3-sys is newer than sqlx accepts,
  and only one may link SQLite. It goes away at the end of this phase.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/retention.rs

@@ -149,10 +149,10 @@ mod tests {
         // age_key 0 means "never recorded" -- not the same as "infinitely old".
     }
 
-    #[test]
-    fn query_never_offers_a_file_anyone_starred_and_prefers_ones_everyone_read() {
+    #[tokio::test]
+    async fn query_never_offers_a_file_anyone_starred_and_prefers_ones_everyone_read() {
         // One file serves both subscribers, so it takes both of them to release it.
-        let db = Db::memory().unwrap();
+        let db = Db::memory().await.unwrap();
         db.exec_for_test(
             "INSERT INTO users (id, name, is_admin) VALUES (1,'ray',1),(2,'sam',0);
              INSERT INTO subscriptions (user_id, feed_id) VALUES (1,'f'),(2,'f');
@@ -185,9 +185,9 @@ mod tests {
         );
     }
 
-    #[test]
-    fn prune_keeps_entries_that_still_have_a_file() {
-        let db = Db::memory().unwrap();
+    #[tokio::test]
+    async fn prune_keeps_entries_that_still_have_a_file() {
+        let db = Db::memory().await.unwrap();
         db.exec_for_test(
             "INSERT INTO users (id, name, is_admin) VALUES (1,'ray',1);
              INSERT INTO entry_state (user_id, feed_id, guid, flagged) VALUES (1,'f','flagged',1);