perf: index the link-cache prune, evict chats with no live prompt

Two things the 300 s sweep did the hard way:

- The link cache is pruned by `created_at` (`DELETE FROM link_cache WHERE
  created_at < ?`) and had no index on it, so every sweep scanned the whole
  table — every post sent inside the TTL window, which is up to a week of
  them — while the `url` primary key served none of it. A new migration
  (appended; migration 1 is frozen and already shipped) creates the index, and
  the upgrade test now asserts it exists after an upgrade.
- `ChatStore::prune_expired` only ever *looked* at chats that had an expired
  edit-before-forward record, so a chat with no prompt at all — the common
  case: every chat that ever sent a message or ran a command — stayed in the
  cache and in the per-chat lock map for the process lifetime. The candidate
  set now includes chats holding no records, which is what the eviction below
  was written for; the DB keeps the row, so the next use costs one SELECT
  (pinned by a new test that also shows the durable settings come back).

Deliberately *not* done: skipping the write in `ChatStore::set` when the state
is unchanged. Comparing against the cached copy would skip a serialize plus a
blocking DB round trip for a no-op update — but every one of the 13 `update`
callers mutates something, so the no-op case is a user repeating an identical
command, and the same comparison would also skip the write that repairs a row
whose earlier write failed. A rare saving against a rare repair, and the write
is what makes the cache a cache rather than a source of truth.

Verified: the new eviction test fails without the candidate change (checked by
reverting it) and passes with it; 118 bot tests and 91 x-media tests pass.

`cargo fmt --check`, `cargo clippy --workspace --all-targets --locked -- -D
warnings` and `cargo test --workspace --locked` clean.

crates/xmedia-bot/src/db.rs

@@ -139,6 +139,11 @@ const MIGRATIONS: &[&str] = &[
     // written by its former holder — which used to duplicate a send or drop
     // the new holder's retry state, silently.
     "ALTER TABLE tasks ADD COLUMN lease_token TEXT",
+    // 2: the 300 s sweep prunes the link cache by `created_at`
+    // (`DELETE FROM link_cache WHERE created_at < ?`). Without an index that
+    // is a full scan of every post sent inside the TTL window — up to a week
+    // of them — on every sweep; the `url` primary key cannot serve it.
+    "CREATE INDEX IF NOT EXISTS idx_link_cache_created_at ON link_cache(created_at)",
 ];
 
 /// Brings an existing database up to [`MIGRATIONS`]. Idempotent: a database
@@ -285,6 +290,18 @@ mod tests {
                 )
                 .unwrap();
             assert_eq!(payload, "{\"chat_id\":1}", "rows survive the upgrade");
+            // The link-cache prune's index arrives with the migrations (the
+            // baseline schema has none): without it every sweep scans the
+            // whole table.
+            let index: i64 = conn
+                .query_row(
+                    "SELECT COUNT(*) FROM sqlite_master \
+                     WHERE type = 'index' AND name = 'idx_link_cache_created_at'",
+                    [],
+                    |row| row.get(0),
+                )
+                .unwrap();
+            assert_eq!(index, 1, "the migration's index must exist");
             Ok(())
         })
         .await

crates/xmedia-bot/src/state.rs

@@ -131,18 +131,24 @@ impl ChatStore {
     pub async fn prune_expired(&self, ttl: Duration) -> Vec<(i64, i64)> {
         let now = unix_now();
         let ttl_secs = ttl.as_secs() as i64;
-        // Chats that may have an expired record, from a cache snapshot; the
-        // pruning itself re-reads and writes under the per-chat lock below
-        // (see the eviction note). Takes no lock of its own, so a chat
-        // appearing later is simply picked up by the next sweep.
+        // Chats worth looking at, from a cache snapshot: the ones with an
+        // expired record, plus the ones holding no record at all. The latter
+        // used to be left alone for the process lifetime — every chat that ever
+        // sent a message or ran a command stayed in the cache and in the
+        // per-chat lock map — even though a chat with no live prompt is exactly
+        // what the eviction below is for. The pruning itself re-reads and
+        // writes under the per-chat lock below; taking no lock here means a
+        // chat appearing later is simply picked up by the next sweep.
         let candidates: Vec<i64> = {
             let cache = self.cache.lock();
             cache
                 .iter()
                 .filter(|(_, data)| {
-                    data.edit_message
-                        .values()
-                        .any(|entry| entry.created_at + ttl_secs <= now)
+                    data.edit_message.is_empty()
+                        || data
+                            .edit_message
+                            .values()
+                            .any(|entry| entry.created_at + ttl_secs <= now)
                 })
                 .map(|(chat_id, _)| *chat_id)
                 .collect()
@@ -265,6 +271,58 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn an_idle_chat_is_evicted_and_its_state_reloads() {
+        let dir = tempfile::tempdir().unwrap();
+        let pool = crate::db::open_store(dir.path().join("e.db").to_str().unwrap()).unwrap();
+        let store = ChatStore::new(pool);
+        // Durable settings and no prompt at all: this chat used to sit in the
+        // cache (and in the per-chat lock map) for the process lifetime,
+        // because the sweep only ever looked at chats with an *expired* record.
+        store
+            .update(9, |data| {
+                data.forward_channel_id = Some(-100);
+                data.message_format.insert("twitter".into(), "{url}".into());
+            })
+            .await;
+        assert!(store.cache.lock().contains_key(&9));
+
+        let removed = store.prune_expired(Duration::from_secs(60)).await;
+
+        assert!(removed.is_empty(), "nothing had expired");
+        assert!(
+            !store.cache.lock().contains_key(&9),
+            "a chat with no live prompt must leave the cache"
+        );
+        assert!(!store.locks.lock().contains_key(&9), "…and its lock");
+        // The DB kept the row, so the next use reloads everything it held.
+        let data = store.get(9).await;
+        assert_eq!(data.forward_channel_id, Some(-100));
+        assert_eq!(
+            data.message_format.get("twitter").map(String::as_str),
+            Some("{url}")
+        );
+    }
+
+    #[tokio::test]
+    async fn a_live_prompt_keeps_its_chat_cached() {
+        let dir = tempfile::tempdir().unwrap();
+        let pool = crate::db::open_store(dir.path().join("k.db").to_str().unwrap()).unwrap();
+        let store = ChatStore::new(pool);
+        store
+            .update(10, |data| {
+                data.edit_message.insert(1, edit_entry(10, unix_now()));
+            })
+            .await;
+
+        store.prune_expired(Duration::from_secs(3600)).await;
+
+        assert!(
+            store.cache.lock().contains_key(&10),
+            "a live prompt holds its chat in the cache"
+        );
+    }
+
     #[tokio::test]
     async fn prune_eviction_keeps_the_persisted_state() {
         // Every record expires → the chat is evicted from the cache; the