Add playing handicap, 6-digit login code, and game UX improvements

- Playing handicap: computed at game creation (WHS formula), stored on
  game_players alongside per-hole strokes allocation; displayed on game
  summary with HC card and / markers on scorecard (only when SI defined);
  live game shows +N per player card when SI exists, HC total otherwise;
  new game confirmation previews computed HC per player

- Login: 6-digit code sent alongside magic link in every auth email;
  check_email page has auto-advancing digit inputs with paste support;
  new POST /auth/verify-code route handles code verification

- Game view: Score/Card toggle fixed with position:fixed to always show
  above nav bar on mobile; swipe on score view changes holes, swipe on
  scorecard switches tabs; dirty tracking prevents saving unmodified holes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/database.py

@@ -121,6 +121,9 @@ async def init_db():
             "ALTER TABLE game_players ADD COLUMN tee_id TEXT REFERENCES course_tees(id)",
             "ALTER TABLE course_holes ADD COLUMN stroke_index INTEGER",
             "ALTER TABLE games ADD COLUMN carryover INTEGER NOT NULL DEFAULT 1",
+            "ALTER TABLE game_players ADD COLUMN playing_handicap INTEGER",
+            "ALTER TABLE game_players ADD COLUMN strokes_allocation TEXT",
+            "ALTER TABLE magic_link_tokens ADD COLUMN code TEXT",
         ]:
             try:
                 await db.execute(migration)