Games: cross-device sync + overlap colour-blend

Two game polish items:

- Word Search: overlapping cells now multiply-blend the crossing words' colours
  (deepening to a darker shade with readable text) instead of the newest colour
  stomping the rest — matches the new interlocking grids.

- Cross-device game-state sync (signed-in): per-puzzle progress + stats now
  follow you between devices. New game_state table; server-side merge on every
  save so two devices converge regardless of push order, tailored per game:
  * Word Search → UNION of finds (monotonic; can't un-find), earliest start,
    best completion time.
  * Word → furthest-progress wins (terminal beats in-progress; more guesses
    beats fewer) — picks one device's game whole, never splices guesses.
  Stats (streak/distribution/best) derived server-side from the synced states,
  so they're consistent instead of per-device counters. Endpoints GET/PUT
  /api/games/state + GET /api/games/stats (signed-in; size-capped). Frontend is
  local-first: games paint instantly from localStorage, then reconcile in the
  background; both game components push debounced on each move and adopt the
  merge. Conflict handling unit-tested + an API two-device convergence test.

235→ tests + 11 vitest green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/db.py

@@ -300,6 +300,16 @@ CREATE TABLE IF NOT EXISTS daily_puzzles (
     UNIQUE (puzzle_date, game, variant)
 );
 
+CREATE TABLE IF NOT EXISTS game_state (
+    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    game TEXT NOT NULL,            -- 'word' | 'wordsearch'
+    variant TEXT NOT NULL,         -- '5'|'6' | 'small'|'med'|'large'
+    puzzle_date TEXT NOT NULL,
+    state_json TEXT NOT NULL,      -- per-puzzle progress; merged server-side on save
+    updated_at TEXT NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (user_id, game, variant, puzzle_date)
+);
+
 CREATE TABLE IF NOT EXISTS user_follows (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,