Index the file rows with a file on disk (#165)

Counting a person's files through enclosures_entry, once the count was of their own feeds only, read
every file row of their 146 feeds to find the few with a file: 58 ms of a feed list, more than the
37-50 ms the count over every row of the table had taken. Production holds 15 files among many
thousand rows. A partial index on the rows with a path holds just those, and serves the healthcheck's
count and the Downloaded filter too.

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

src/db.rs

@@ -65,6 +65,10 @@ async fn create_missing(orm: &sea_orm::DatabaseConnection) -> Result<()> {
     }
     for sql in [
         "CREATE INDEX IF NOT EXISTS enclosures_entry ON enclosures (feed_id, guid)",
+        // The few rows with a file on disk, 15 of the server's many thousand: counting a person's
+        // files through enclosures_entry read every file row of their feeds, 58 ms a feed list
+        // (#165). Also what the healthcheck's count and the Downloaded filter look for.
+        "CREATE INDEX IF NOT EXISTS enclosures_on_disk ON enclosures (feed_id, guid) WHERE path IS NOT NULL",
         "CREATE UNIQUE INDEX IF NOT EXISTS users_name_lower ON users (lower(name))",
     ] {
         orm.execute_unprepared(sql).await.with_context(|| sql.to_owned())?;