fix(merge): carry moves/hints and per-variant best moves on account merge

The account merge predated the recent statistics additions and silently dropped
them:
- account_stats.moves and hints_used were not summed into the primary (the
  secondary's row is read, then deleted), so the secondary's lifetime moves and
  hints were lost.
- account_best_move was not handled at all; the secondary is only tombstoned
  (not deleted), so its per-variant best moves lingered on the dead account and
  never reached the merged statistics screen.

mergeStats now also sums moves + hints_used; a new mergeBestMoves folds the
secondary's best moves into the primary keeping the higher-scoring play per
variant (the rule the per-game upsert uses), then deletes the secondary's rows.
Profile fields were already correct (hint wallet summed, paid_account ORed; no
new accounts columns since).

Tests: TestAccountMergeCore extended — moves/hints summed, best moves merged
(higher score per variant kept), secondary rows cleaned up. Docs: ARCHITECTURE §4.

docs/ARCHITECTURE.md

@@ -203,7 +203,8 @@ arrive from a platform rather than completing a mandatory registration).
   guest is promoted to durable, clearing `is_guest`).
 - **Merge** retires the account that owns the linked identity into the **current**
   account, in a single transaction (`internal/accountmerge`): statistics summed
-  (max points kept), the hint wallet summed, `paid_account` ORed, identities
+  (counters incl. moves/hints added, max points kept, and the per-variant best moves
+  merged keeping the higher-scoring play), the hint wallet summed, `paid_account` ORed, identities
   repointed, games / chat / complaints transferred, friends and blocks
   de-duplicated (friendships keep the strongest status accepted>pending>declined),
   pending invitations/codes dropped, and the secondary kept as an **audit