feat(backup): daily server and user backups, restore by section (backend)

Admins switch automatic backups on, pick the time and the retention
(default 03:00, 3 days). Each run writes a gzip snapshot of the whole
database (VACUUM INTO, consistent while others write) and one MindSift
backup ZIP per active user below $DATA_DIR/backups, then deletes files
past the retention. A restart after the time catches up on that day.
Admins list, download and trigger runs; users list, download and restore
their own backups.

Import and restore can be limited to sections (notes, tasks, lists,
inbox, themes): restore only empties the chosen sections. Export and
import now carry custom themes. File names are checked against a fixed
pattern, so no other file can be fetched through the API.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/models.go

@@ -292,6 +292,10 @@ type SystemSettings struct {
 	// metadata stay blocked
 	AllowPrivateWebhooks bool `json:"allowPrivateWebhooks"`
 	SMTPConfiguredViaEnv bool `json:"smtpConfiguredViaEnv,omitempty"`
+	// Automatic backups: on/off, daily time (HH:MM, server time zone) and how many days to keep them
+	BackupEnabled       bool   `json:"backupEnabled"`
+	BackupTime          string `json:"backupTime"`
+	BackupRetentionDays int    `json:"backupRetentionDays"`
 }
 
 // UnmarshalJSON accepts smtpPort as text or number: older clients sent a number, which made the whole
@@ -532,6 +536,7 @@ type FullExport struct {
 	Documents  []Document   `json:"documents"`
 	Lists      []SharedList `json:"lists"`
 	TaskLists  []TaskList   `json:"taskLists,omitempty"`
+	Themes     []UserTheme  `json:"themes,omitempty"`
 }
 
 type SharedFolder struct {