Add topic/flavor categorization and category browsing

- New taxonomy module: single source of truth for 6 topics x 5 flavors,
  shared by the LLM response schema (enum-constrained) and validation.
- Classifier now assigns one topic + one flavor per article; json_schema
  enums force valid values, with coercion as a safety net.
- article_scores gains topic/flavor columns via an idempotent migration.
- New 'list-category' command to browse by topic and/or flavor, ranked by
  composite score.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/db.py

@@ -56,6 +56,8 @@ CREATE TABLE IF NOT EXISTS article_scores (
     accepted INTEGER,
     reason_code TEXT,
     reason_text TEXT,
+    topic TEXT,
+    flavor TEXT,
     model_name TEXT,
     scored_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
@@ -102,4 +104,17 @@ def connect(db_path: Path | str) -> sqlite3.Connection:
 
 def init_db(conn: sqlite3.Connection) -> None:
     conn.executescript(SCHEMA)
+    _migrate(conn)
     conn.commit()
+
+
+def _migrate(conn: sqlite3.Connection) -> None:
+    """Add columns introduced after the initial schema to existing databases.
+
+    CREATE TABLE IF NOT EXISTS never alters an existing table, so new columns
+    need an explicit, idempotent ALTER guarded by the current column set.
+    """
+    cols = {row["name"] for row in conn.execute("PRAGMA table_info(article_scores)")}
+    for column in ("topic", "flavor"):
+        if column not in cols:
+            conn.execute(f"ALTER TABLE article_scores ADD COLUMN {column} TEXT")