Admin: Daily Word pool curation (lookup + add/remove)

First games admin tool. A "Games" tab in the operator console for the Daily Word
answer pool.
* Lookup: is a word real (in the guess dictionary), the right length (5/6), and
  already in the pool — instant as you type.
* Add: appends to the pool, enforcing the invariant (alpha · 5/6 letters · in the
  guess dict) so the daily answer is always guessable. Remove: drops admin-added
  words (curated static ones stay).
* Additions persist in a new word_pool table (survives redeploys, unlike the
  baked-in JSON); the daily picker reads static pool ∪ DB additions. Guess dicts
  shipped with the package (goodnews/data/words-5/6.json) for server-side
  validation. Admin-gated endpoints + tests.

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

goodnews/db.py

@@ -260,6 +260,13 @@ CREATE TABLE IF NOT EXISTS feedback_replies (
 );
 CREATE INDEX IF NOT EXISTS idx_feedback_replies_fid ON feedback_replies(feedback_id);
 
+CREATE TABLE IF NOT EXISTS word_pool (
+    word TEXT NOT NULL,
+    variant TEXT NOT NULL,       -- '5' | '6'
+    created_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (variant, word)
+);
+
 CREATE TABLE IF NOT EXISTS daily_puzzles (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     puzzle_date TEXT NOT NULL,