feat(account): restore the confirm deeplink token (domain layer)

Re-apply the deeplink backend deferred out of PR1a: migration 00006 adds
email_confirmations.purpose + link_token_hash (hand-edited jet), each issued code
now carries an opaque 256-bit token (only its SHA-256 stored), and ConfirmByToken
resolves a token to a login (confirm + clear guest) or a link (attach when free,
signal merge when owned elsewhere). issueCode now embeds the /app/#/confirm/<token>
link in the email. The confirm endpoint, gateway RPC and SPA route follow.

backend/internal/postgres/jet/backend/model/email_confirmations.go

@@ -21,4 +21,6 @@ type EmailConfirmations struct {
 	Attempts       int16
 	ConsumedAt     *time.Time
 	CreatedAt      time.Time
+	Purpose        string
+	LinkTokenHash  *string
 }

backend/internal/postgres/jet/backend/table/email_confirmations.go

@@ -25,6 +25,8 @@ type emailConfirmationsTable struct {
 	Attempts       postgres.ColumnInteger
 	ConsumedAt     postgres.ColumnTimestampz
 	CreatedAt      postgres.ColumnTimestampz
+	Purpose        postgres.ColumnString
+	LinkTokenHash  postgres.ColumnString
 
 	AllColumns     postgres.ColumnList
 	MutableColumns postgres.ColumnList
@@ -74,9 +76,11 @@ func newEmailConfirmationsTableImpl(schemaName, tableName, alias string) emailCo
 		AttemptsColumn       = postgres.IntegerColumn("attempts")
 		ConsumedAtColumn     = postgres.TimestampzColumn("consumed_at")
 		CreatedAtColumn      = postgres.TimestampzColumn("created_at")
-		allColumns           = postgres.ColumnList{ConfirmationIDColumn, AccountIDColumn, EmailColumn, CodeHashColumn, ExpiresAtColumn, AttemptsColumn, ConsumedAtColumn, CreatedAtColumn}
-		mutableColumns       = postgres.ColumnList{AccountIDColumn, EmailColumn, CodeHashColumn, ExpiresAtColumn, AttemptsColumn, ConsumedAtColumn, CreatedAtColumn}
-		defaultColumns       = postgres.ColumnList{AttemptsColumn, CreatedAtColumn}
+		PurposeColumn        = postgres.StringColumn("purpose")
+		LinkTokenHashColumn  = postgres.StringColumn("link_token_hash")
+		allColumns           = postgres.ColumnList{ConfirmationIDColumn, AccountIDColumn, EmailColumn, CodeHashColumn, ExpiresAtColumn, AttemptsColumn, ConsumedAtColumn, CreatedAtColumn, PurposeColumn, LinkTokenHashColumn}
+		mutableColumns       = postgres.ColumnList{AccountIDColumn, EmailColumn, CodeHashColumn, ExpiresAtColumn, AttemptsColumn, ConsumedAtColumn, CreatedAtColumn, PurposeColumn, LinkTokenHashColumn}
+		defaultColumns       = postgres.ColumnList{AttemptsColumn, CreatedAtColumn, PurposeColumn}
 	)
 
 	return emailConfirmationsTable{
@@ -91,6 +95,8 @@ func newEmailConfirmationsTableImpl(schemaName, tableName, alias string) emailCo
 		Attempts:       AttemptsColumn,
 		ConsumedAt:     ConsumedAtColumn,
 		CreatedAt:      CreatedAtColumn,
+		Purpose:        PurposeColumn,
+		LinkTokenHash:  LinkTokenHashColumn,
 
 		AllColumns:     allColumns,
 		MutableColumns: mutableColumns,

backend/internal/postgres/migrations/00006_email_confirm_deeplink.sql

@@ -0,0 +1,27 @@
+-- Add the confirm deeplink token and the confirmation purpose to email_confirmations.
+-- purpose tags what verifying the code/token does (email login, identity link, email
+-- change, or account deletion); link_token_hash holds the SHA-256 hex of the one-click
+-- deeplink token (only the hash is stored, mirroring the session-token model).
+-- Expand-contract: both columns are additive. purpose gets a NOT NULL DEFAULT so
+-- existing rows fill in; link_token_hash is nullable with a partial-unique index. A
+-- backend image rollback stays DB-safe — older code simply ignores the new columns. The
+-- table shape changes, so the generated go-jet model for this table is regenerated.
+
+-- +goose Up
+ALTER TABLE backend.email_confirmations
+    ADD COLUMN purpose text NOT NULL DEFAULT 'link',
+    ADD COLUMN link_token_hash text;
+ALTER TABLE backend.email_confirmations
+    ADD CONSTRAINT email_confirmations_purpose_chk
+        CHECK ((purpose = ANY (ARRAY['login'::text, 'link'::text, 'change'::text, 'delete'::text])));
+CREATE UNIQUE INDEX email_confirmations_link_token_hash_key
+    ON backend.email_confirmations (link_token_hash)
+    WHERE link_token_hash IS NOT NULL;
+
+-- +goose Down
+DROP INDEX IF EXISTS backend.email_confirmations_link_token_hash_key;
+ALTER TABLE backend.email_confirmations
+    DROP CONSTRAINT IF EXISTS email_confirmations_purpose_chk;
+ALTER TABLE backend.email_confirmations
+    DROP COLUMN IF EXISTS link_token_hash,
+    DROP COLUMN IF EXISTS purpose;