Fix alembic.ini missing + entrypoint CWD safety

Update shared submodule (adds alembic.ini for migrations).
Use subshell for alembic so CWD is never affected by cd shared,
preventing cascading config path errors if migration fails.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

entrypoint.sh

@@ -13,7 +13,7 @@ fi
 # Run DB migrations only if RUN_MIGRATIONS=true (blog service only)
 if [[ "${RUN_MIGRATIONS:-}" == "true" ]]; then
   echo "Running Alembic migrations..."
-  cd shared && alembic upgrade head && cd /app
+  (cd shared && alembic upgrade head)
 fi
 
 # Clear Redis page cache on deploy