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>

Cargo.toml

@@ -18,7 +18,8 @@ percent-encoding = "2.3.2"
 quick-xml = { version = "0.42.0", features = ["escape-html"] }
 reqwest = { version = "0.13.5", default-features = false, features = ["rustls", "http2", "gzip", "stream", "json", "charset", "system-proxy"] }
 rss = "2.1.1"
-rusqlite = { version = "0.40.2", features = ["bundled"] }
+rusqlite = { version = "0.39", features = ["bundled"] }
+sea-orm = { version = "~2.0.3", default-features = false, features = ["sqlx-sqlite", "sqlx-postgres", "runtime-tokio-rustls", "macros", "with-json", "schema-sync", "sqlite-use-returning-for-3_35"] }
 serde = { version = "1.0.229", features = ["derive"] }
 serde_json = "1.0.151"
 tokio = { version = "1.53.1", features = ["rt-multi-thread", "macros", "fs", "io-util", "net", "sync", "time", "signal"] }