Fix shared upload permissions for capability-restricted LTX backend

docs/ARCHITECTURE.md

@@ -27,6 +27,10 @@ outputs are mounted read-only. Only configured input/output roots are readable;
 traversal and symlinks escaping those roots are rejected. ffmpeg/ffprobe create
 thumbnails, dimensions and extracted frames on CPU; they are not inference runtimes.
 Network protocols are disabled for media inspection. HTTP Range supports seeking.
+Uploads, copied assets and thumbnails are mode 0640. Provision the writable input
+folder with a shared backend group and mode 2750 (setgid), so new files inherit
+that group. Both service identities must be able to traverse/read the shared
+folder; a root UID with dropped capabilities cannot bypass ordinary permissions.
 
 Projects: initial preview retains the upstream browser-local project storage.
 Projects are specific to this browser/origin, not multiuser or cross-device synced.

docs/ATHENA_DEPLOYMENT.md

@@ -7,7 +7,8 @@ The additional `deploy/compose.athena.yaml` uses host networking **only for this
 frontend**, binds its web server to `127.0.0.1:8118`, and connects to Deck at
 `http://127.0.0.1:8120`. This avoids changing Deck's existing network or relying on
 its current Docker IP. No Docker socket, GPU devices or inference packages are
-mounted into the GUI. CPU and RAM limits: 2 CPUs / 1 GiB.
+mounted into the GUI. CPU and RAM limits: 2 CPUs / 1 GiB. GUI identity: UID 1000 / GID 0;
+this supplies shared-file read access, not root UID or Linux capabilities.
 
 Filtered Deck inventory labels:
 
@@ -17,7 +18,9 @@ Filtered Deck inventory labels:
 Media mounts:
 
 - `/data/video/ltx-desktop/LTXDesktop/remote-inputs/deskweb` → writable inputs.
-  A new isolated directory owned by UID/GID 1000; no recursive permission changes.
+  A new isolated directory owned by UID 1000 / GID 0, mode 2750; no changes to other LTX directories.
+  Files use mode 0640. The existing LTX backend runs with GID 0 and dropped
+  capabilities, so group-read/traverse permission is required even for its root UID.
 - `/data/video/ltx-desktop/LTXDesktop/outputs` → read-only outputs.
 
 Backend-visible input path: `/data/LTXDesktop/remote-inputs/deskweb`.

docs/VALIDATION.md

@@ -33,3 +33,21 @@ synthetic text upload and readback through shared input storage (test file remov
 Deck's actual Unix-socket helper inventory returns `ltx-deskweb` as `running`.
 Existing Medium, TTS, WireGuard and Deck remain running; LTX remains stopped.
 Real video generation was not requested or tested in this deployment.
+
+## Shared-input permission fix — 2026-09-29
+
+A real I2V request exposed a cross-container permission error: private uploads
+(0600 in a 0750 UID/GID-1000 directory) were invisible to LTX's UID/GID-0 process
+because its container drops all Linux capabilities. Same bind mount/path did not
+imply read permission. The API reported this as “Image file not found”.
+
+Dedicated DeskWEB input directory now uses UID 1000 / GID 0, mode 2750; uploads,
+asset copies and thumbnails use 0640. Athena GUI runs as 1000:0 with capabilities
+still dropped. Only existing DeskWEB-owned input files had their permissions
+corrected; no other media directory, backend container or GPU service was changed.
+
+Validation: build/typecheck and all 8 tests pass; integration tests now assert
+0640 on uploads, copies (including a 0600 source) and thumbnails. On Athena the
+reported image's existence/read permission was verified from inside LTX without
+opening its content. A fresh synthetic upload was also checked from LTX and then
+removed. Only the GUI was recreated; real image generation remains a user retry.