perf(storage): composite (profile_id, created_at DESC) index on transcripts

Migration v15 adds a composite index covering the dominant transcripts
query path:

    SELECT ... FROM transcripts
    WHERE profile_id = ? ORDER BY created_at DESC LIMIT ?

Previously SQLite had to choose between idx_transcripts_profile_id
(filter by profile, then in-memory sort by date) and idx_transcripts_created
(scan dates and filter on profile). Both work fine at hundreds of rows
and degrade past a few thousand.

`migration_v15_creates_profile_created_index` asserts (a) the index exists
and (b) `EXPLAIN QUERY PLAN` shows the planner picks it for the canonical
profile-scoped, date-ordered list query.

Test count assertions in `test_migrations_run_on_empty_db` and
`test_migrations_idempotent` bumped 14 → 15.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/storage/src/migrations.rs

@@ -463,6 +463,20 @@ const MIGRATIONS: &[(i64, &str, &str)] = &[
         ALTER TABLE transcripts ADD COLUMN llm_tags TEXT NOT NULL DEFAULT '';
         "#,
     ),
+    (
+        15,
+        "transcripts: composite (profile_id, created_at DESC) index",
+        r#"
+        -- Performance index for the dominant transcripts query path:
+        --   WHERE profile_id = ? ORDER BY created_at DESC LIMIT ?
+        -- The standalone idx_transcripts_profile_id and idx_transcripts_created
+        -- forced SQLite to either filter by profile then sort, or scan the date
+        -- index and filter — fine at hundreds of rows, painful past a few thousand.
+        -- A composite index covers both predicates in one ordered seek.
+        CREATE INDEX IF NOT EXISTS idx_transcripts_profile_created
+            ON transcripts(profile_id, created_at DESC);
+        "#,
+    ),
 ];
 
 /// Split SQL into individual statements, respecting BEGIN...END trigger blocks.
@@ -612,7 +626,7 @@ mod tests {
             .fetch_one(&pool)
             .await
             .unwrap();
-        assert_eq!(count, 14);
+        assert_eq!(count, 15);
 
         sqlx::query("INSERT INTO settings (key, value) VALUES ('test', 'value')")
             .execute(&pool)
@@ -631,7 +645,7 @@ mod tests {
             .fetch_one(&pool)
             .await
             .unwrap();
-        assert_eq!(count, 14);
+        assert_eq!(count, 15);
     }
 
     #[tokio::test]
@@ -1124,4 +1138,48 @@ mod tests {
             .expect("query");
         assert_eq!(auto, 0, "pre-existing completed rows must default to 0");
     }
+
+    #[tokio::test]
+    async fn migration_v15_creates_profile_created_index() {
+        let pool = SqlitePoolOptions::new()
+            .max_connections(1)
+            .connect("sqlite::memory:")
+            .await
+            .expect("pool");
+        run_migrations(&pool).await.expect("migrate");
+
+        // Index exists by name.
+        let names: Vec<String> = sqlx::query_scalar(
+            "SELECT name FROM sqlite_master \
+             WHERE type = 'index' AND tbl_name = 'transcripts'",
+        )
+        .fetch_all(&pool)
+        .await
+        .expect("read indexes");
+        assert!(
+            names.iter().any(|n| n == "idx_transcripts_profile_created"),
+            "expected composite (profile_id, created_at) index, got {names:?}",
+        );
+
+        // Query planner picks the composite index for the dominant
+        // profile-scoped, date-ordered list query. EXPLAIN QUERY PLAN
+        // returns (id, parent, notused, detail) — we want detail.
+        let plan_rows = sqlx::query(
+            "EXPLAIN QUERY PLAN \
+             SELECT id FROM transcripts \
+             WHERE profile_id = ? ORDER BY created_at DESC LIMIT 50",
+        )
+        .bind(crate::DEFAULT_PROFILE_ID)
+        .fetch_all(&pool)
+        .await
+        .expect("explain");
+        let plan: Vec<String> = plan_rows
+            .iter()
+            .map(|r| r.get::<String, _>("detail"))
+            .collect();
+        assert!(
+            plan.iter().any(|row| row.contains("idx_transcripts_profile_created")),
+            "planner should use the composite index, got plan: {plan:?}",
+        );
+    }
 }