feat(store): invitations table + create/peek/claim methods

Stage-1 email onboarding: Mathias mints an invite, the recipient claims
it to set a Dex password. Invitations exist before their user, so the
table carries no user_id FK and is deliberately outside RLS — the
32-byte crypto-random token is the capability (single-use, time-boxed).

ClaimInvitation consumes atomically (UPDATE ... WHERE used_at IS NULL
... RETURNING) so concurrent claims of one token can't both succeed.
PeekInvitation validates the link for the form without consuming it.

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

internal/adapters/store/migrations/009_invitations.down.sql

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