Content dedup on both sides, stored checksums, spool-free zips, upload stats

- sha256+crc32 hashed during upload streaming; unique index per album
- duplicate content returns the existing photo (race-safe via 23505)
- client hashes locally (WebCrypto) and skips the transfer entirely for
  content the album already has
- zip downloads stream S3->response directly using the stored crc32;
  pre-hash photos spool once and self-heal (crc via zip, sha via reprocess)
- upload UI: overall progress bar, bytes, live speed, ETA

migrations/0002_content_hash.sql

@@ -0,0 +1,10 @@
+-- Content hashes: sha256 powers duplicate-upload detection (same content in
+-- the same album is returned instead of copied); crc32 lets zip downloads
+-- stream originals straight from S3 without a local spool pass.
+-- Both are null for photos uploaded before this migration; they self-heal on
+-- reprocess (sha256 + crc32) and on zip download (crc32).
+alter table photos add column sha256 text;
+alter table photos add column crc32 bigint;
+
+create unique index photos_album_sha_uidx on photos (album_id, sha256)
+    where sha256 is not null;