refactor(hive-c0re): shared additive-migration helper in db

db::apply_migrations runs ALTER lists and ignores duplicate-column
errors (turn_stats' pattern); approvals, operator_questions, broker
reminders, and scheduled_prompts drop their hand-rolled
pragma_table_info guards. broker's acked_at migration stays bespoke —
its backfill must only run when the column was just created

hive-c0re/src/scheduled_prompts.rs

@@ -186,12 +186,11 @@ impl ScheduledPrompts {
         conn.execute_batch(SCHEMA)
             .context("apply scheduled_prompts schema")?;
         // Migration: add paused_at_unix to existing databases.
-        // Silently ignores "duplicate column name" errors so this is
-        // idempotent across daemon restarts on already-migrated DBs.
-        let _ = conn.execute(
-            "ALTER TABLE scheduled_prompts ADD COLUMN paused_at_unix INTEGER",
-            [],
-        );
+        crate::db::apply_migrations(
+            &conn,
+            "scheduled_prompts",
+            &["ALTER TABLE scheduled_prompts ADD COLUMN paused_at_unix INTEGER"],
+        )?;
         // Migration: recreate the due-rows index to also exclude paused
         // rows. `CREATE INDEX IF NOT EXISTS` won't update an existing
         // index's WHERE clause, so we drop + recreate on every open.