deploy: treat an empty .last_good_tag the same as a missing one (#156)

cat file 2>/dev/null || echo latest only falls back when cat itself fails
(nonzero exit, i.e. the file is missing) -- a file that EXISTS but is EMPTY
makes cat succeed with empty output, so PREV_TAG became "" instead of
"latest". That "" failed the emptiness check further down and exited 1 --
AFTER git pull + docker compose up -d had already succeeded. Worse: exiting
there skips the Health check step (Gitea Actions doesn't run later steps
after a failure), and Health check is the ONLY step that ever writes a real
value to .last_good_tag. Self-perpetuating: once the file went empty, every
future deploy failed the same way forever, with the app itself deploying
fine underneath it every time (confirmed against run 611: deploy log shows
all three images pulled/recreated/started at 66bbf5b, and /health
self-reports that exact GIT_SHA, baked into the image at build time --
two independent signals, same commit).

${VAR:-default} covers empty and unset in one expansion, matching the
fallback behavior the surrounding comment already documented as intended.

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

.gitea/workflows/deploy.yml

@@ -133,7 +133,20 @@ jobs:
           # has confirmed the tag it names actually answered /health. A
           # fresh VM with no file yet falls back to :latest, same escape
           # hatch as a manual `up -d` with no TAG set.
-          PREV_TAG=$(cat /opt/drb/.last_good_tag 2>/dev/null || echo latest)
+          #
+          # server-26#156: `cat missing-file || echo latest` only falls back
+          # when cat itself fails (nonzero exit) -- a file that EXISTS but is
+          # EMPTY (the state this file was found in, 2026-09-20) makes cat
+          # succeed with empty output, so PREV_TAG became "" instead of
+          # "latest". That "" then failed the emptiness check below and
+          # exited 1 -- AFTER git pull + up -d had already succeeded -- which
+          # skips the Health check step entirely (later steps don't run after
+          # a failure), and Health check is the ONLY thing that ever writes a
+          # real value here. Self-perpetuating: every deploy failed the same
+          # way forever, with the app itself deploying fine underneath it.
+          # ${VAR:-default} covers empty AND unset in one expansion.
+          PREV_TAG=$(cat /opt/drb/.last_good_tag 2>/dev/null)
+          PREV_TAG="${PREV_TAG:-latest}"
           echo "PREV_TAG=$PREV_TAG"
 
           # Deploy THIS commit's images, not :latest. Overlapping runs are