feat(mailboxes): plads til Gmails udløser og dens set-liste

Kolonnerne, ikke flytningen. Gmail-handlerne læser stadig user_settings, og en
migration, der tømte den tabel før de var skrevet om, ville koble en fungerende
postkasse fra. Det additive først; flytningen når koden på den anden side er klar
til den, og den ligger som PENDING_gmail_move.sql.txt uden for .sql-mønstret,
så den ikke kan køre ved et uheld.

Udløseren fik sin egen kolonne frem for at blive pakket ind i label med et kolon.
En kolonne, der rummer to ting adskilt af et tegn, er en kolonne, nogen læser
forkert senere — og en etiket med kolon i navnet ville gøre det på første dag.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/migrations/PENDING_gmail_move.sql.txt

@@ -0,0 +1,48 @@
+-- NOT A MIGRATION YET. Deliberately outside the .sql glob so it cannot run.
+--
+-- This is the data move for task #23. It must not be applied until the Gmail
+-- handlers read the mailboxes table instead of user_settings: it empties the
+-- settings row, and running it early would disconnect a working mailbox.
+--
+-- Rename to 0016_gmail_is_a_mailbox.sql when internal/httpapi/gmail_handlers.go
+-- has been rewritten.
+
+-- The values move across still sealed. Both tables seal with the same key and
+-- store the same string, so copying the ciphertext verbatim is not a shortcut
+-- around the encryption — it is the encryption, untouched. That also means this
+-- migration does not need the key, which a migration has no business holding.
+--
+-- `username` is the address the account reports, and it is what makes the unique
+-- index meaningful. It may be empty on rows that never got that far; two such rows
+-- cannot exist for one person anyway, because the settings table could only hold
+-- one.
+INSERT INTO mailboxes (
+    id, user_id, kind, name, host, username, password, folder,
+    refresh_token, access_token, expires_at, label, trigger_kind, seen,
+    last_uid, last_sync_at, created_at
+)
+SELECT
+    lower(hex(randomblob(16))),
+    s.user_id,
+    'gmail',
+    COALESCE(json_extract(s.values_json, '$.email'), 'Gmail'),
+    '',
+    COALESCE(json_extract(s.values_json, '$.email'), ''),
+    '',
+    'INBOX',
+    COALESCE(json_extract(s.values_json, '$.refresh_token'), ''),
+    COALESCE(json_extract(s.values_json, '$.access_token'), ''),
+    COALESCE(json_extract(s.values_json, '$.expires_at'), 0),
+    COALESCE(json_extract(s.values_json, '$.label'), ''),
+    COALESCE(json_extract(s.values_json, '$.trigger'), 'starred'),
+    COALESCE(json_extract(s.values_json, '$.seen'), '[]'),
+    0,
+    0,
+    unixepoch()
+FROM user_settings s
+WHERE s.scope = 'gmail'
+  AND COALESCE(json_extract(s.values_json, '$.refresh_token'), '') <> '';
+
+-- Emptied, not deleted: the row may hold nothing else today, but the table is
+-- shared and a DELETE here would be a habit worth not forming.
+UPDATE user_settings SET values_json = '{}' WHERE scope = 'gmail';