Add DB-backed model change polling and startup pruning

crates/yaak-models/migrations/20260216000000_model-changes.sql

@@ -0,0 +1,12 @@
+CREATE TABLE model_changes
+(
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    model         TEXT                                                    NOT NULL,
+    model_id      TEXT                                                    NOT NULL,
+    change        TEXT                                                    NOT NULL,
+    update_source TEXT                                                    NOT NULL,
+    payload       TEXT                                                    NOT NULL,
+    created_at    DATETIME DEFAULT (STRFTIME('%Y-%m-%d %H:%M:%f', 'NOW')) NOT NULL
+);
+
+CREATE INDEX idx_model_changes_created_at ON model_changes (created_at);