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/storage/import_markdown.go

@@ -28,12 +28,34 @@ const (
 // ImportPlan is everything an import would create. It is built completely (and validated) before
 // anything is written, so the preview shows exactly what the import does.
 type ImportPlan struct {
-	Folders   []string         `json:"-"`
-	Docs      []importDoc      `json:"-"`
-	TaskLists []importTaskList `json:"-"`
-	Lists     []importList     `json:"-"`
-	Inbox     []importTask     `json:"-"`
-	Skipped   []ImportSkip     `json:"skipped"`
+	Folders   []string           `json:"-"`
+	Docs      []importDoc        `json:"-"`
+	TaskLists []importTaskList   `json:"-"`
+	Lists     []importList       `json:"-"`
+	Inbox     []importTask       `json:"-"`
+	Themes    []domain.UserTheme `json:"-"`
+	Skipped   []ImportSkip       `json:"skipped"`
+}
+
+// Only keeps the chosen sections of the plan (see NormalizeSections).
+func (p *ImportPlan) Only(sections map[string]bool) *ImportPlan {
+	out := &ImportPlan{Skipped: p.Skipped}
+	if sections[SectionNotes] {
+		out.Folders, out.Docs = p.Folders, p.Docs
+	}
+	if sections[SectionTasks] {
+		out.TaskLists = p.TaskLists
+	}
+	if sections[SectionLists] {
+		out.Lists = p.Lists
+	}
+	if sections[SectionInbox] {
+		out.Inbox = p.Inbox
+	}
+	if sections[SectionThemes] {
+		out.Themes = p.Themes
+	}
+	return out
 }
 
 type ImportSkip struct {
@@ -50,6 +72,7 @@ type ImportSummary struct {
 	Lists     int          `json:"lists"`
 	ListItems int          `json:"listItems"`
 	Inbox     int          `json:"inbox"`
+	Themes    int          `json:"themes"`
 	Skipped   []ImportSkip `json:"skipped"`
 	Applied   bool         `json:"applied"`
 }
@@ -85,7 +108,7 @@ func (p *ImportPlan) skip(path, reason string) {
 }
 
 func (p *ImportPlan) Summary(applied bool) ImportSummary {
-	sum := ImportSummary{Folders: len(p.Folders), Notes: len(p.Docs), Lists: len(p.Lists), Inbox: len(p.Inbox), Skipped: p.Skipped, Applied: applied}
+	sum := ImportSummary{Folders: len(p.Folders), Notes: len(p.Docs), Lists: len(p.Lists), Inbox: len(p.Inbox), Themes: len(p.Themes), Skipped: p.Skipped, Applied: applied}
 	for _, tl := range p.TaskLists {
 		if tl.title != "" {
 			sum.TaskLists++
@@ -416,6 +439,7 @@ func PlanMindSiftJSONImport(data []byte) (*ImportPlan, error) {
 		}
 		plan.Lists = append(plan.Lists, il)
 	}
+	plan.Themes = exp.Themes
 	plan.Folders = sortedKeys(folders)
 	return plan, nil
 }
@@ -484,6 +508,11 @@ func (s *Store) ApplyImport(userID string, plan *ImportPlan) (ImportSummary, err
 			}
 		}
 	}
+	for _, th := range plan.Themes {
+		if _, err := s.SaveUserTheme(userID, "", th.Name, th.CSS); err != nil {
+			plan.skip(th.Name, "Theme: "+err.Error())
+		}
+	}
 	return plan.Summary(true), nil
 }