fix(cache): web UI can read what the display service caches again (#593)

* fix(cache): web UI can read what the display service caches again

ledmatrix-web.service carried CacheDirectory=ledmatrix. With User= set to
the installing user, systemd re-owns /var/cache/ledmatrix and everything
in it to that user and its primary group whenever the directory's owner
differs -- for a directory root created, on the first start. That erased
the root:ledmatrix setgid layout the installers set up, so every file the
display service (root) wrote afterwards was root:root 0660 and unreadable
by the web interface:

  WARNING - Permission denied loading cache for display_current_state ...

Since #547 install_service.sh renders the web unit from the template, so
every fresh install hit this. Measured on one rig: 392 unreadable files,
and the web UI's display status, on-demand state and plugin health empty.

Existing installs only receive `git pull`, never a reinstalled unit, so
the fix for them is in the code the root display service runs:

- DiskCache.set gives each file the directory's group (when the directory
  is group-writable) and 0660 on the open descriptor before the rename,
  independent of setgid. This also closes a window where a fresh file was
  visible as mkstemp's 0600.
- DiskCache.share_existing_files repairs files an older version left
  behind, once per process from the cleanup thread. It works through
  O_NOFOLLOW descriptors and skips hard links and other users' files: the
  directory is writable by the web user, and root must not be steered
  into changing a file outside it.

For new installs, the web unit drops CacheDirectory=/CacheDirectoryMode=,
and install_web_service.sh stops replacing an existing directory's
ledmatrix group with the user's group.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* fix(web): on-demand and current-display status read the display's latest state

Found testing the cache-permission fix on a rig: once the web interface
could read display_on_demand_state at all, /display/on-demand/status kept
answering "active" for over 100 seconds while the file on disk said
"idle". Both status routes read the display service's keys through the
web process's memory tier, which serves the first copy it read for the
full max_age (120s). Read them with memory_ttl=0, as every other
cross-process reader (plugin health/metrics, the on-demand mailbox)
already does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* fix(install): re-group the cache dir whenever the web user is outside its group

install_web_service.sh replaced an existing cache directory's group only
when it was root's. A directory in any other group the web user is not a
member of -- root:ledmatrix, for a user who is not in ledmatrix -- was left
alone, and every file root wrote there stayed unreadable to the web
interface. Replace the group whenever the installing user is not in it.

A directory whose group the user is already in (ledmatrix, or the user's
own group where CacheDirectory= left it) is still left as it is: re-grouping
a working directory strands the files already in it on the old group.

When the group does change and root-owned JSON files carrying the old group
are present, try-restart ledmatrix.service so DiskCache.share_existing_files
re-groups them through its symlink- and hard-link-safe path, rather than a
recursive chgrp.

Verified under WSL's systemd for seven directory states (user group,
ledmatrix member, ledmatrix non-member with and without root files,
root:root, missing, unnamed gid); the previous version left the non-member
case unchanged.

Addresses CodeRabbit review on #593.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/display.py

@@ -138,7 +138,11 @@ def get_on_demand_status():
     """Return the current on-demand display state."""
     try:
         cache = _ensure_cache_manager()
-        state = cache.get('display_on_demand_state', max_age=120)
+        # memory_ttl=0: the display service writes this key, so only the file
+        # is current. This process's memory tier would keep serving the first
+        # copy it read for the full max_age -- "active" for two minutes after
+        # the display had already stopped.
+        state = cache.get('display_on_demand_state', max_age=120, memory_ttl=0)
         if state is None:
             state = {
                 'active': False,
@@ -304,7 +308,8 @@ def get_current_display_status():
     """
     try:
         cache = _ensure_cache_manager()
-        state = cache.get('display_current_state', max_age=120)
+        # memory_ttl=0: written by the display service; see get_on_demand_status.
+        state = cache.get('display_current_state', max_age=120, memory_ttl=0)
         if state is None:
             state = {
                 'mode': None,