feat(slice-2): accounts domain — request_code/verify/get_account (INV-2/INV-3/INV-6)

Email-canonical get-or-create, peppered-hash one-time codes with 10-min TTL, single-use,
5-attempt cap, and 60s resend cooldown; uniform new-or-known (no enumeration, §6.6). All
identity rules in the domain layer once (INV-6); scenario + invariant tests green.

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

backend/app/domains/accounts/__init__.py

@@ -0,0 +1,31 @@
+"""accounts domain — identity: account records, one-time-code issue/verify, account lookup.
+
+Owns INV-2 (email canonical key) and INV-3 (one-time codes handled like secrets). Knows
+nothing about storefronts (that is the storefronts domain, SLICE-3). Imported via this
+package surface only (§6.2).
+"""
+from __future__ import annotations
+
+from .errors import (
+    AccountsError,
+    CodeExhausted,
+    CodeExpired,
+    CodeMismatch,
+    InvalidEmail,
+    ResendCooldown,
+)
+from .models import Account
+from .service import get_account, request_code, verify
+
+__all__ = [
+    "Account",
+    "AccountsError",
+    "InvalidEmail",
+    "ResendCooldown",
+    "CodeMismatch",
+    "CodeExpired",
+    "CodeExhausted",
+    "request_code",
+    "verify",
+    "get_account",
+]