feat: classifier wrapper with retry, timeout, schema-default fallback

chat/db/migrations/0002_classifier_failures.sql

@@ -0,0 +1,8 @@
+CREATE TABLE classifier_failures (
+    id INTEGER PRIMARY KEY,
+    kind TEXT NOT NULL,
+    model TEXT NOT NULL,
+    raw_text TEXT,
+    attempt_count INTEGER NOT NULL,
+    created_at TEXT NOT NULL DEFAULT (datetime('now'))
+);