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

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+DROP TABLE IF EXISTS user_identities;

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

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+-- Migration 004: the Dex-subject → tapir-user map (ADR-012 Stage 1, multi-user).
+-- A Dex-authenticated subject is the login identity; the tapir user_id (UUID) is
+-- what every user-owned, force-RLS table keys off. This table is the bridge:
+-- resolve subject → user_id here (auth plumbing, pre-scope), THEN scope all data
+-- access by that id via the store's withUser helper.
+--
+-- INTENTIONALLY NOT RLS-ENABLED. The forced-RLS isolation (migration 003) guards
+-- the user-OWNED data tables. user_identities holds no user data — only an opaque
+-- (dex_subject ↔ user_id) pair — and must be readable BEFORE a user_id is known
+-- (that lookup is what yields the id used to set tapir.current_user_id). Putting
+-- RLS here would be a chicken-and-egg deadlock (you'd need the GUC to read the row
+-- that tells you the GUC). Data isolation lives on the user-owned tables, not here.
+CREATE TABLE user_identities (
+    dex_subject TEXT PRIMARY KEY,
+    user_id     UUID NOT NULL UNIQUE REFERENCES users(id) ON DELETE CASCADE,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+COMMENT ON TABLE user_identities IS
+    'Dex subject -> tapir user_id map. Auth plumbing, deliberately NOT RLS-enabled '
+    '(no user data; must be read pre-scope to resolve the id used for RLS). '
+    'ON DELETE CASCADE so deleting a user cleans up its identity mapping.';