feat(save-transfer): add saveutil CLI and token-gated import endpoint

Adds two complementary paths for transferring character save data between
Erupe instances without breaking the SHA-256 integrity check system:

- `cmd/saveutil/`: admin CLI with `import`, `export`, `grant-import`, and
  `revoke-import` subcommands. Direct DB access; no server running required.
- `POST /v2/characters/{id}/import`: player-facing API endpoint gated behind
  a one-time token issued by `saveutil grant-import` (default TTL 24 h).
  Token is validated and consumed atomically to prevent TOCTOU races.
- Migration `0013_save_transfer`: `savedata_import_token` and
  `savedata_import_token_expiry` columns on `characters` table.
- Both paths decompress incoming savedata and recompute the SHA-256 hash
  server-side, so the integrity check remains valid after import.
- README documents both methods and the per-character hash-reset workaround.

Closes #183.

server/migrations/sql/0013_save_transfer.sql

@@ -0,0 +1,6 @@
+-- Save transfer tokens: one-time admin-granted permission for a character
+-- to receive an imported save via the API endpoint.
+-- NULL means no import is pending for this character.
+ALTER TABLE characters
+    ADD COLUMN IF NOT EXISTS savedata_import_token        TEXT,
+    ADD COLUMN IF NOT EXISTS savedata_import_token_expiry TIMESTAMPTZ;