Add scheduled and manual backups

Backups are .tar.gz archives of a consistent database snapshot (VACUUM
INTO) plus the images folder. They run on a cron schedule, daily at 3:00
by default, and the newest N scheduled ones are kept (7 by default).
Manual backups are never pruned.

Settings → Backups offers schedule presets or a custom cron expression,
retention, "Back up now", and download/delete for each backup. A small
built-in parser handles standard 5-field cron syntax with names, ranges,
steps and @daily-style shorthands.

Backups go to CANON_BACKUP_DIR (default data/backups). Compose mounts a
separate canon-backups volume at /backups, overridable with
CANON_BACKUP_PATH.

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

internal/server/server.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/cdnutter/canon/internal/auth"
+	"github.com/cdnutter/canon/internal/backup"
 	"github.com/cdnutter/canon/internal/config"
 	"github.com/cdnutter/canon/internal/imgcache"
 	"github.com/cdnutter/canon/internal/providers"
@@ -35,6 +36,7 @@ type Server struct {
 	cfg       config.Config
 	store     *store.Store
 	images    *imgcache.Cache
+	backups   *backup.Manager
 	providers *providers.Registry
 	log       *slog.Logger
 
@@ -48,11 +50,12 @@ type Server struct {
 	previewClient *http.Client
 }
 
-func New(cfg config.Config, st *store.Store, images *imgcache.Cache, log *slog.Logger, version string) (*Server, error) {
+func New(cfg config.Config, st *store.Store, images *imgcache.Cache, backups *backup.Manager, log *slog.Logger, version string) (*Server, error) {
 	s := &Server{
 		cfg:        cfg,
 		store:      st,
 		images:     images,
+		backups:    backups,
 		log:        log,
 		assetVer:   version,
 		loginLimit: auth.NewLimiter(10, 15*time.Minute),
@@ -125,6 +128,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /admin/lists/{id}", s.requireUser(s.handleAdminList))
 	mux.HandleFunc("GET /admin/settings", s.requireUser(s.handleAdminSettings))
 	mux.HandleFunc("GET /admin/preview", s.requireUser(s.handleAdminPreview))
+	mux.HandleFunc("GET /admin/backups/{name}", s.requireUser(s.handleBackupDownload))
 
 	// Admin JSON API
 	api := func(pattern string, h http.HandlerFunc) { mux.HandleFunc(pattern, s.requireAPI(h)) }
@@ -147,6 +151,10 @@ func (s *Server) Handler() http.Handler {
 	api("DELETE /api/settings/avatar", s.apiDeleteAvatar)
 	api("POST /api/integrations/test", s.apiTestIntegration)
 	api("PUT /api/account", s.apiUpdateAccount)
+	api("GET /api/backups", s.apiBackups)
+	api("POST /api/backups", s.apiCreateBackup)
+	api("PUT /api/backups/schedule", s.apiBackupSchedule)
+	api("DELETE /api/backups/{name}", s.apiDeleteBackup)
 
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) { s.notFound(w, r) })