chore(docker): mount ~/Pictures read-write so file ops work; document

The first phase-11 file op (inline rename) returns EROFS today
because docker-compose mounts ~/Pictures read-only by default.
Lightroom-style file operations (rename, move, discard-pile empty)
all need to mutate the filesystem, so the right default is :rw.

Flips both the backend and worker mounts to :rw with an inline
comment explaining the trade-off, and adds a "Photo directory
mounts and permissions" section to the README that:
- States the default is now :rw
- Explains exactly which endpoints fail under :ro (rename, empty
  discard pile, future move/copy)
- Notes the implication: Mulita has full write access to whatever
  host directory ends up at /host/Pictures, same trust model as
  Lightroom's catalog folder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -24,7 +24,11 @@ services:
     volumes:
       - ./mulita.yml:/app/config/mulita.yml:ro
       - ${PHOTO_DIRS:-./photos}:/photos:rw
-      - ~/Pictures:/host/Pictures:ro
+      # NOTE: read-write — file operations (rename, move, discard,
+      # empty discard pile) need to mutate the filesystem. Flip to :ro
+      # if you want a strict read-only library; the rename / move /
+      # delete endpoints will then return EROFS.
+      - ~/Pictures:/host/Pictures:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
       - db_data:/data/db
@@ -50,7 +54,8 @@ services:
     volumes:
       - ./mulita.yml:/app/config/mulita.yml:ro
       - ${PHOTO_DIRS:-./photos}:/photos:rw
-      - ~/Pictures:/host/Pictures:ro
+      # See backend service for the rationale on :rw.
+      - ~/Pictures:/host/Pictures:rw
       - thumbs_data:/data/thumbs
       - proxies_data:/data/proxies
       - db_data:/data/db