feat(store): user_identities map + UserBySubject/RegisterUser (ADR-012)

Add the Dex-subject -> tapir-user_id bridge for multi-user Stage 1.

Migration 004 creates user_identities (dex_subject PK, user_id UNIQUE FK
ON DELETE CASCADE). It is intentionally NOT RLS-enabled: it holds no user
data and must be readable BEFORE a user_id is known (the lookup is what
yields the id used to set tapir.current_user_id). RLS here would be a
chicken-and-egg deadlock; data isolation stays on the user-owned tables.

UserBySubject resolves subject -> user_id as a plain pool query (pre-scope,
no withUser). RegisterUser generates the UUID app-side (stdlib crypto/rand,
no new dep) so the forced-RLS WITH CHECK (id = GUC) passes, then inserts the
users row via withUser(newID) and the identity row in the same transaction.
Re-registration of a subject errors with ErrSubjectRegistered.

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

internal/adapters/store/migrations/004_user_identities.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS user_identities;