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/models.py

@@ -0,0 +1,12 @@
+"""accounts — domain models (SD-0001 §6.3)."""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Account:
+    """A person's identity on the platform. Email is the canonical key (INV-2)."""
+
+    id: int
+    email: str