Add mobile save sheet with same-origin photo downloads.

Phones cannot land downloads in the photo library, so the gallery opens a share sheet for a cacheable preview while streaming via a dedicated download endpoint and recording preview sizes.

Co-authored-by: Cursor <cursoragent@cursor.com>

photo-api/internal/httpapi/sizes.go

@@ -0,0 +1,64 @@
+package httpapi
+
+import (
+	"context"
+	"log"
+	"sync"
+
+	"git.azzamo.net/Michilis/Spanglish/photo-api/internal/store"
+)
+
+// statConcurrency bounds the one-off Stat burst below; a gallery of ~70
+// photos then costs a handful of round-trips against S3, once ever.
+const statConcurrency = 8
+
+// fillPreviewSizes measures the preview objects of photos whose
+// preview_size_bytes is still unset and writes the result back, updating the
+// slice in place.
+//
+// The worker records the size when it generates the preview, so this only
+// ever fires for rows that predate the column (or arrived via `photo-api
+// sync`). It is a self-healing cache fill rather than a migration step: the
+// first gallery view after deploy pays for one Stat per photo, every view
+// after that reads the stored value. Errors are logged and swallowed — a
+// missing size costs the save sheet its row label, which is not worth
+// failing a gallery response over.
+func (s *Server) fillPreviewSizes(ctx context.Context, photos []store.Photo) {
+	var (
+		wg   sync.WaitGroup
+		sem  = make(chan struct{}, statConcurrency)
+		mu   sync.Mutex
+		size = make(map[string]int64, len(photos))
+	)
+	for _, p := range photos {
+		if p.PreviewKey == "" || p.PreviewSizeBytes > 0 {
+			continue
+		}
+		wg.Add(1)
+		go func(p store.Photo) {
+			defer wg.Done()
+			sem <- struct{}{}
+			defer func() { <-sem }()
+			n, err := s.storage.Stat(ctx, p.PreviewKey)
+			if err != nil {
+				log.Printf("preview size %s: %v", p.PreviewKey, err)
+				return
+			}
+			mu.Lock()
+			size[p.ID] = n
+			mu.Unlock()
+		}(p)
+	}
+	wg.Wait()
+	if len(size) == 0 {
+		return
+	}
+	for i := range photos {
+		if n, ok := size[photos[i].ID]; ok {
+			photos[i].PreviewSizeBytes = n
+			if err := s.db.SetPreviewSize(ctx, photos[i].ID, n); err != nil {
+				log.Printf("store preview size %s: %v", photos[i].ID, err)
+			}
+		}
+	}
+}