fix(account): dedupe colliding identities on merge, journaling to the dossier

An account merge blanket-reassigned all of the secondary's identities to the primary,
so merging two accounts that each had a confirmed email (or telegram/vk) left the
survivor with two identities of one kind — which the profile and the retention dossier
both treat as singular (the profile showed an arbitrary one; a later change-email
journaled only one). The merge now keeps the primary's identity and journals the
secondary's colliding one to retained_identities (reason=merge) before dropping it, so
the survivor has one identity per kind and the absorbed credential still lands in the
legal dossier.

- migration 00008: widen retained_identities.reason CHECK to admit 'merge'
  (expand-contract — Up only widens the set, so an image rollback stays DB-safe).
- accountmerge: dedupeIdentities + retainMergedIdentity before the identity reassign.
- inttest TestAccountMergeDedupesEmail; docs ARCHITECTURE §4 + retention.go reason note.

backend/internal/account/retention.go

@@ -21,9 +21,11 @@ import (
 const RetentionTTL = 2 * 365 * 24 * time.Hour
 
 // Reasons recorded on a retained_identities row: what detached the credential from its
-// account. The row is written just before the live identities row is removed, preserving
-// the legal dossier (which email/vk/tg was linked, and when) even as the identity frees
-// for reuse. See docs/ARCHITECTURE.md §9.1.
+// account (unlink / email change / account deletion here; an account merge that drops a
+// same-kind colliding identity writes reason "merge" from the accountmerge package). The
+// row is written just before the live identities row is removed, preserving the legal
+// dossier (which email/vk/tg was linked, and when) even as the identity frees for reuse.
+// See docs/ARCHITECTURE.md §9.1.
 const (
 	retainUnlink = "unlink"
 	retainChange = "change"