Cap MySQL statement cache and connection lifetime in storyteller-web

Production hit MySQL error 1461 (max_prepared_stmt_count = 16382), causing
fast 500s on /v1/prompt/batch and other endpoints. sqlx caches up to 100
prepared statements per connection by default, so 7 replicas x 50
connections could hold ~35k statements, more than twice the server cap.

- statement_cache_capacity defaults to 25 (MYSQL_STATEMENT_CACHE_CAPACITY),
  bringing the ceiling to ~8.75k at current scale.
- max_lifetime defaults to 10 minutes (MYSQL_MAX_LIFETIME_SECONDS), down from
  sqlx's 30, so statements orphaned by cancelled queries are released sooner.

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

crates/service/web/storyteller_web/src/configs/connect_to_database.rs

@@ -1,10 +1,11 @@
+use std::str::FromStr;
 use std::time::Duration;
 
 use log::info;
 
 use errors::AnyhowResult;
 use shared_env_var_config::mysql::env_get_mysql_connection_string_or_default;
-use sqlx::mysql::MySqlPoolOptions;
+use sqlx::mysql::{MySqlConnectOptions, MySqlPoolOptions};
 use sqlx::MySqlPool;
 
 pub async fn connect_to_database() -> AnyhowResult<MySqlPool> {
@@ -20,20 +21,37 @@ pub async fn connect_to_database() -> AnyhowResult<MySqlPool> {
   // shorter timeout sheds load and surfaces the problem instead of hiding it behind long waits.
   let acquire_timeout_seconds = easyenv::get_env_num::<u64>("MYSQL_ACQUIRE_TIMEOUT_SECONDS", 10)?;
 
+  // NB: Every cached prepared statement stays open on the server until its connection evicts it
+  // or closes, and the server caps the total across ALL clients (`max_prepared_stmt_count`,
+  // 16382 on the managed DB). sqlx's default of 100 per connection let `replicas *
+  // MYSQL_MAX_CONNECTIONS * 100` blow past that cap, failing new prepares with MySQL error 1461.
+  // Keep `replicas * MYSQL_MAX_CONNECTIONS * MYSQL_STATEMENT_CACHE_CAPACITY` well under it.
+  let statement_cache_capacity = easyenv::get_env_num::<usize>("MYSQL_STATEMENT_CACHE_CAPACITY", 25)?;
+
+  // NB: Recycling connections releases any server-side statements orphaned by cancelled queries
+  // (eg. a client disconnect between prepare and close). sqlx's default lifetime is 30 minutes.
+  let max_lifetime_seconds = easyenv::get_env_num::<u64>("MYSQL_MAX_LIFETIME_SECONDS", 600)?;
+
   // NB: Log the settings before building/connecting so a connect failure has context. We do NOT
   // log the connection string (it contains credentials).
   info!(
-    "Building MySQL pool: max_connections={}, min_connections={}, acquire_timeout_seconds={}",
+    "Building MySQL pool: max_connections={}, min_connections={}, acquire_timeout_seconds={}, \
+      statement_cache_capacity={}, max_lifetime_seconds={}",
     max_connections, min_connections, acquire_timeout_seconds,
+    statement_cache_capacity, max_lifetime_seconds,
   );
 
+  let connect_options = MySqlConnectOptions::from_str(&db_connection_string)?
+      .statement_cache_capacity(statement_cache_capacity);
+
   let pool_options = MySqlPoolOptions::new()
       .max_connections(max_connections)
       .min_connections(min_connections)
-      .acquire_timeout(Duration::from_secs(acquire_timeout_seconds));
+      .acquire_timeout(Duration::from_secs(acquire_timeout_seconds))
+      .max_lifetime(Some(Duration::from_secs(max_lifetime_seconds)));
 
   let pool = pool_options
-      .connect(&db_connection_string)
+      .connect_with(connect_options)
       .await?;
 
   Ok(pool)