feat(db): retention schema for account deletion (migration 00007)

Additive/expand-contract: new append-only retained_identities table (the legal
dossier of every detached credential, keyed by account_id, TTL'd by detached_at)
plus nullable accounts columns last_login_at/last_login_ip (cold-load stamp) and
deleted_at/deleted_display_name (tombstone + retained real name). Regenerates the
go-jet models for accounts + retained_identities only.

backend/internal/postgres/migrations/00007_account_deletion_retention.sql

@@ -0,0 +1,47 @@
+-- Account deletion as legal retention (not erasure). Two additive pieces.
+--
+-- retained_identities is an append-only journal of every credential detached from an
+-- account — on unlink, email change, or account deletion (reason). It preserves the legal
+-- dossier (which email/vk/tg was linked, and when) while the live identities row is
+-- removed, so the (kind, external_id) frees for a new account to reuse. No unique
+-- constraint and no kind CHECK: the same credential may recur across accounts and events,
+-- and the log stays robust to future identity kinds. detached_at drives the retention TTL.
+--
+-- accounts gains: last_login_at / last_login_ip (stamped on a cold app-load, throttled),
+-- deleted_at (the tombstone marker), and deleted_display_name (the real name retained for
+-- the admin dossier after the live display_name is scrubbed to the anonymised label).
+--
+-- Expand-contract: everything is additive (a new table plus nullable columns), so a
+-- backend image rollback stays DB-safe — older code simply ignores them. The accounts
+-- table shape changes, so its generated go-jet model is regenerated.
+
+-- +goose Up
+CREATE TABLE backend.retained_identities (
+    retained_id uuid NOT NULL,
+    account_id uuid NOT NULL,
+    kind text NOT NULL,
+    external_id text NOT NULL,
+    confirmed boolean DEFAULT false NOT NULL,
+    linked_at timestamp with time zone NOT NULL,
+    detached_at timestamp with time zone DEFAULT now() NOT NULL,
+    reason text NOT NULL,
+    CONSTRAINT retained_identities_pkey PRIMARY KEY (retained_id),
+    CONSTRAINT retained_identities_reason_chk
+        CHECK ((reason = ANY (ARRAY['unlink'::text, 'change'::text, 'delete'::text])))
+);
+CREATE INDEX retained_identities_account_id_idx ON backend.retained_identities (account_id);
+CREATE INDEX retained_identities_detached_at_idx ON backend.retained_identities (detached_at);
+
+ALTER TABLE backend.accounts
+    ADD COLUMN last_login_at timestamp with time zone,
+    ADD COLUMN last_login_ip text,
+    ADD COLUMN deleted_at timestamp with time zone,
+    ADD COLUMN deleted_display_name text;
+
+-- +goose Down
+ALTER TABLE backend.accounts
+    DROP COLUMN last_login_at,
+    DROP COLUMN last_login_ip,
+    DROP COLUMN deleted_at,
+    DROP COLUMN deleted_display_name;
+DROP TABLE backend.retained_identities;