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/db.rs

@@ -22,6 +22,26 @@ use rusqlite::Connection;
 /// means something is genuinely wedged, not ordinary contention.
 const BUSY_TIMEOUT: Duration = Duration::from_secs(5);
 
+/// Apply additive, idempotent migrations: run each statement and
+/// ignore `duplicate column name` errors (sqlite has no
+/// `ADD COLUMN IF NOT EXISTS`; try-and-ignore is the portable path —
+/// same pattern as hive-ag3nt's `turn_stats`). Any other error
+/// propagates. Fits plain `ALTER TABLE ADD COLUMN` (new columns must
+/// carry a default or tolerate NULL) and `IF NOT EXISTS` index DDL;
+/// migrations with creation-conditional backfills (broker's
+/// `acked_at`) stay bespoke in their store.
+pub fn apply_migrations(conn: &Connection, subsystem: &str, statements: &[&str]) -> Result<()> {
+    for stmt in statements {
+        if let Err(e) = conn.execute_batch(stmt) {
+            if e.to_string().contains("duplicate column name") {
+                continue;
+            }
+            return Err(e).with_context(|| format!("{subsystem} migration failed: {stmt}"));
+        }
+    }
+    Ok(())
+}
+
 /// Open a connection to the sqlite file at `path`, creating the parent
 /// directory if needed. `subsystem` labels error contexts (`"broker"`,
 /// `"approvals"`, …).